diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -106,6 +106,7 @@ add_executable(${PROGRAM_NAME} WIN32
   src/ringbuffer.cpp
   src/ringbuffer.cpp
   src/indexbuffer.cpp
+  src/linindexbuffer.cpp
   src/readonlybuffer.cpp
   src/framebufferseries.cpp
   src/numberformatbox.cpp
diff --git a/src/framebuffer.h b/src/framebuffer.h
--- a/src/framebuffer.h
+++ b/src/framebuffer.h
@@ -61,4 +61,24 @@ class WFrameBuffer : public ResizableBuf
     virtual void clear() = 0;
 };
 
+/**
+ * Abstract base class for X buffers.
+ *
+ * These buffers only contain increasing or equal (to previous) values.
+ */
+class XFrameBuffer : public ResizableBuffer
+{
+public:
+    enum Index {OUT_OF_RANGE = -1};
+
+    /**
+     * Finds index for given value.
+     *
+     * If given value is bigger than max or smaller than minimum
+     * returns `OUT_OF_RANGE`. If it's in between values, smaller
+     * index is returned (not closer one).
+     */
+    virtual int findIndex(double value) const = 0;
+};
+
 #endif // FRAMEBUFFER_H
diff --git a/src/framebufferseries.cpp b/src/framebufferseries.cpp
--- a/src/framebufferseries.cpp
+++ b/src/framebufferseries.cpp
@@ -20,75 +20,64 @@
 #include <math.h>
 #include "framebufferseries.h"
 
-FrameBufferSeries::FrameBufferSeries(const FrameBuffer* buffer)
+FrameBufferSeries::FrameBufferSeries(const XFrameBuffer* x, const FrameBuffer* y)
 {
-    xAsIndex = true;
-    _xmin = 0;
-    _xmax = 1;
-    _buffer = buffer;
+    _x = x;
+    _y = y;
+
     int_index_start = 0;
-    int_index_end = _buffer->size();
+    int_index_end = _y->size();
 }
 
-void FrameBufferSeries::setXAxis(bool asIndex, double xmin, double xmax)
+void FrameBufferSeries::setX(const XFrameBuffer* x)
 {
-    xAsIndex = asIndex;
-    _xmin = xmin;
-    _xmax = xmax;
+    _x = x;
 }
 
 size_t FrameBufferSeries::size() const
 {
-    return int_index_end - int_index_start;
+    return int_index_end - int_index_start + 1;
 }
 
 QPointF FrameBufferSeries::sample(size_t i) const
 {
     i += int_index_start;
-    if (xAsIndex)
-    {
-        return QPointF(i, _buffer->sample(i));
-    }
-    else
-    {
-        return QPointF(i * (_xmax - _xmin) / _buffer->size() + _xmin, _buffer->sample(i));
-    }
+    return QPointF(_x->sample(i), _y->sample(i));
 }
 
 QRectF FrameBufferSeries::boundingRect() const
 {
     QRectF rect;
-    auto yLim = _buffer->limits();
+    auto yLim = _y->limits();
+    auto xLim = _x->limits();
     rect.setBottom(yLim.start);
     rect.setTop(yLim.end);
-    if (xAsIndex)
-    {
-        rect.setLeft(0);
-        rect.setRight(size());
-    }
-    else
-    {
-        rect.setLeft(_xmin);
-        rect.setRight(_xmax);
-    }
+    rect.setLeft(xLim.start);
+    rect.setRight(xLim.end);
+
     return rect.normalized();
 }
 
 void FrameBufferSeries::setRectOfInterest(const QRectF& rect)
 {
-    if (xAsIndex)
+    int_index_start = _x->findIndex(rect.left());
+    int_index_end = _x->findIndex(rect.right());
+
+    if (int_index_start == XFrameBuffer::OUT_OF_RANGE)
     {
-        int_index_start = floor(rect.left())-1;
-        int_index_end = ceil(rect.right())+1;
+        int_index_start = 0;
     }
-    else
+    else if (int_index_start > 0)
     {
-        double xsize = _xmax - _xmin;
-        size_t bsize = _buffer->size();
-        int_index_start =  floor(bsize * (rect.left()-_xmin) / xsize)-1;
-        int_index_end = ceil(bsize * (rect.right()-_xmin) / xsize)+1;
+        int_index_start -= 1;
     }
 
-    int_index_start = std::max(int_index_start, 0);
-    int_index_end = std::min((int) _buffer->size(), int_index_end);
+    if (int_index_end == XFrameBuffer::OUT_OF_RANGE)
+    {
+        int_index_end = _x->size()-1;
+    }
+    else if (int_index_end < (int)_x->size()-1)
+    {
+        int_index_end += 1;
+    }
 }
diff --git a/src/framebufferseries.h b/src/framebufferseries.h
--- a/src/framebufferseries.h
+++ b/src/framebufferseries.h
@@ -35,10 +35,9 @@
 class FrameBufferSeries : public QwtSeriesData<QPointF>
 {
 public:
-    FrameBufferSeries(const FrameBuffer* buffer);
+    FrameBufferSeries(const XFrameBuffer* x, const FrameBuffer* y);
 
-    /// Behavior of X axis
-    void setXAxis(bool asIndex, double xmin, double xmax);
+    void setX(const XFrameBuffer* x);
 
     // QwtSeriesData implementations
     size_t size() const;
@@ -47,10 +46,8 @@ public:
     void setRectOfInterest(const QRectF& rect);
 
 private:
-    const FrameBuffer* _buffer;
-    bool xAsIndex;
-    double _xmin;
-    double _xmax;
+    const XFrameBuffer* _x;
+    const FrameBuffer* _y;
 
     int int_index_start; ///< starting index of "rectangle of interest"
     int int_index_end;   ///< ending index of "rectangle of interest"
diff --git a/src/indexbuffer.cpp b/src/indexbuffer.cpp
--- a/src/indexbuffer.cpp
+++ b/src/indexbuffer.cpp
@@ -1,5 +1,5 @@
 /*
-  Copyright © 2017 Hasan Yavuz Özderya
+  Copyright © 2018 Hasan Yavuz Özderya
 
   This file is part of serialplot.
 
@@ -47,3 +47,15 @@ Range IndexBuffer::limits() const
 {
     return Range{0, _size-1.};
 }
+
+int IndexBuffer::findIndex(double value) const
+{
+    if (value < 0 || value > size() - 1)
+    {
+        return OUT_OF_RANGE;
+    }
+    else
+    {
+        return value;
+    }
+}
diff --git a/src/indexbuffer.h b/src/indexbuffer.h
--- a/src/indexbuffer.h
+++ b/src/indexbuffer.h
@@ -26,15 +26,16 @@
 /// sample value.
 ///
 /// @note This buffer isn't for storing data.
-class IndexBuffer : public ResizableBuffer
+class IndexBuffer : public XFrameBuffer
 {
 public:
     IndexBuffer(unsigned n);
 
-    unsigned size() const;
-    double sample(unsigned i) const;
-    Range limits() const;
-    void resize(unsigned n);
+    unsigned size() const override;
+    double sample(unsigned i) const override;
+    Range limits() const override;
+    void resize(unsigned n) override;
+    int findIndex(double value) const override;
 
 private:
     unsigned _size;
diff --git a/src/linindexbuffer.cpp b/src/linindexbuffer.cpp
--- a/src/linindexbuffer.cpp
+++ b/src/linindexbuffer.cpp
@@ -1,5 +1,5 @@
  /*
-  Copyright © 2017 Hasan Yavuz Özderya
+  Copyright © 2018 Hasan Yavuz Özderya
 
   This file is part of serialplot.
 
@@ -18,12 +18,14 @@
 */
 
 #include <QtGlobal>
+#include <algorithm>
 
 #include "linindexbuffer.h"
 
 LinIndexBuffer::LinIndexBuffer(unsigned n, Range lim)
 {
-    Q_ASSERT(n > 0);
+    // Note that calculation of _step would cause divide by 0
+    Q_ASSERT(n > 1);
 
     _size = n;
     setLimits(lim);
@@ -50,6 +52,18 @@ void LinIndexBuffer::resize(unsigned n)
     setLimits(_limits);         // called to update `_step`
 }
 
+int LinIndexBuffer::findIndex(double value) const
+{
+    if (value < _limits.start || value > _limits.end)
+    {
+        return OUT_OF_RANGE;
+    }
+
+    int r = (value - _limits.start) / _step;
+    // Note: we are limiting return value because of floating point in-accuracies
+    return std::min<int>(std::max<int>(r, 0), (_size-1));
+}
+
 void LinIndexBuffer::setLimits(Range lim)
 {
     _limits = lim;
diff --git a/src/linindexbuffer.h b/src/linindexbuffer.h
--- a/src/linindexbuffer.h
+++ b/src/linindexbuffer.h
@@ -26,17 +26,19 @@
 /// intermediate values are calculated linearly.
 ///
 /// @note This buffer isn't for storing data.
-class LinIndexBuffer : public ResizableBuffer
+class LinIndexBuffer : public XFrameBuffer
 {
 public:
     LinIndexBuffer(unsigned n, Range lim);
     LinIndexBuffer(unsigned n, double min, double max) :
         LinIndexBuffer(n, {min, max}) {};
 
-    unsigned size() const;
-    double sample(unsigned i) const;
-    Range limits() const;
-    void resize(unsigned n);
+    unsigned size() const override;
+    double sample(unsigned i) const override;
+    Range limits() const override;
+    void resize(unsigned n) override;
+    int findIndex(double value) const override;
+
     /// Sets minimum and maximum sample values of the buffer.
     void setLimits(Range lim);
 
diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -169,6 +169,9 @@ MainWindow::MainWindow(QWidget *parent) 
             plotMan, &PlotManager::setYAxis);
 
     connect(&plotControlPanel, &PlotControlPanel::xScaleChanged,
+            &stream, &Stream::setXAxis);
+
+    connect(&plotControlPanel, &PlotControlPanel::xScaleChanged,
             plotMan, &PlotManager::setXAxis);
 
     connect(&plotControlPanel, &PlotControlPanel::plotWidthChanged,
@@ -215,6 +218,9 @@ MainWindow::MainWindow(QWidget *parent) 
     plotControlPanel.setChannelInfoModel(stream.infoModel());
 
     // init scales
+    stream.setXAxis(plotControlPanel.xAxisAsIndex(),
+                    plotControlPanel.xMin(), plotControlPanel.xMax());
+
     plotMan->setYAxis(plotControlPanel.autoScale(),
                       plotControlPanel.yMin(), plotControlPanel.yMax());
     plotMan->setXAxis(plotControlPanel.xAxisAsIndex(),
diff --git a/src/plot.cpp b/src/plot.cpp
--- a/src/plot.cpp
+++ b/src/plot.cpp
@@ -1,5 +1,5 @@
 /*
-  Copyright © 2017 Hasan Yavuz Özderya
+  Copyright © 2018 Hasan Yavuz Özderya
 
   This file is part of serialplot.
 
@@ -91,6 +91,11 @@ Plot::~Plot()
     if (snapshotOverlay != NULL) delete snapshotOverlay;
 }
 
+void Plot::setDispChannels(QVector<const StreamChannel*> channels)
+{
+    zoomer.setDispChannels(channels);
+}
+
 void Plot::setYAxis(bool autoScaled, double yAxisMin, double yAxisMax)
 {
     this->isAutoScaled = autoScaled;
diff --git a/src/plot.h b/src/plot.h
--- a/src/plot.h
+++ b/src/plot.h
@@ -1,5 +1,5 @@
 /*
-  Copyright © 2017 Hasan Yavuz Özderya
+  Copyright © 2018 Hasan Yavuz Özderya
 
   This file is part of serialplot.
 
@@ -50,6 +50,9 @@ public:
     Plot(QWidget* parent = 0);
     ~Plot();
 
+    /// Set displayed channels for value tracking (can be null)
+    void setDispChannels(QVector<const StreamChannel*> channels);
+
 public slots:
     void showGrid(bool show = true);
     void showMinorGrid(bool show = true);
diff --git a/src/plotmanager.cpp b/src/plotmanager.cpp
--- a/src/plotmanager.cpp
+++ b/src/plotmanager.cpp
@@ -19,7 +19,6 @@
 
 #include <algorithm>
 #include <QMetaEnum>
-#include <QtDebug>
 #include "qwt_symbol.h"
 
 #include "plot.h"
@@ -31,9 +30,9 @@ PlotManager::PlotManager(QWidget* plotAr
                          const Stream* stream, QObject* parent) :
     QObject(parent)
 {
+    _stream = stream;
     construct(plotArea, menu);
-    _stream = stream;
-    if (_stream == NULL) return;
+    if (_stream == nullptr) return;
 
     // connect to ChannelInfoModel
     infoModel = _stream->infoModel();
@@ -53,15 +52,15 @@ PlotManager::PlotManager(QWidget* plotAr
     // add initial curves if any?
     for (unsigned int i = 0; i < stream->numChannels(); i++)
     {
-        addCurve(stream->channel(i)->name(), stream->channel(i)->yData());
+        addCurve(stream->channel(i)->name(), stream->channel(i)->xData(), stream->channel(i)->yData());
     }
-
 }
 
 PlotManager::PlotManager(QWidget* plotArea, PlotMenu* menu,
                          Snapshot* snapshot, QObject *parent) :
     QObject(parent)
 {
+    _stream = nullptr;
     construct(plotArea, menu);
 
     setNumOfSamples(snapshot->numSamples());
@@ -70,7 +69,7 @@ PlotManager::PlotManager(QWidget* plotAr
 
     for (unsigned ci = 0; ci < snapshot->numChannels(); ci++)
     {
-        addCurve(snapshot->channelName(ci), snapshot->yData[ci]);
+        addCurve(snapshot->channelName(ci), snapshot->xData[ci], snapshot->yData[ci]);
     }
 
     connect(infoModel, &QAbstractItemModel::dataChanged,
@@ -94,8 +93,6 @@ void PlotManager::construct(QWidget* plo
     // initalize layout and single widget
     isMulti = false;
     scrollArea = NULL;
-    setupLayout(isMulti);
-    addPlotWidget();
 
     // connect to  menu
     connect(menu, &PlotMenu::symbolShowChanged, this, &PlotManager:: setSymbols);
@@ -148,7 +145,7 @@ void PlotManager::onNumChannelsChanged(u
         // add new channels
         for (unsigned int i = oldNum; i < numOfChannels; i++)
         {
-            addCurve(_stream->channel(i)->name(), _stream->channel(i)->yData());
+            addCurve(_stream->channel(i)->name(), _stream->channel(i)->xData(), _stream->channel(i)->yData());
         }
     }
     else if(numOfChannels < oldNum)
@@ -217,8 +214,6 @@ void PlotManager::checkNoVisChannels()
 
 void PlotManager::setMulti(bool enabled)
 {
-    if (enabled == isMulti) return;
-
     isMulti = enabled;
 
     // detach all curves
@@ -239,11 +234,14 @@ void PlotManager::setMulti(bool enabled)
     if (isMulti)
     {
         // add new widgets and attach
+        int i = 0;
         for (auto curve : curves)
         {
             auto plot = addPlotWidget();
             plot->setVisible(curve->isVisible());
+            plot->setDispChannels(QVector<const StreamChannel*>(1, _stream->channel(i)));
             curve->attach(plot);
+            i++;
         }
     }
     else
@@ -251,6 +249,11 @@ void PlotManager::setMulti(bool enabled)
         // add a single widget
         auto plot = addPlotWidget();
 
+        if (_stream != nullptr)
+        {
+            plot->setDispChannels(_stream->allChannels());
+        }
+
         // attach all curves
         for (auto curve : curves)
         {
@@ -332,11 +335,10 @@ Plot* PlotManager::addPlotWidget()
     return plot;
 }
 
-void PlotManager::addCurve(QString title, const FrameBuffer* buffer)
+void PlotManager::addCurve(QString title, const XFrameBuffer* xBuf, const FrameBuffer* yBuf)
 {
     auto curve = new QwtPlotCurve(title);
-    auto series = new FrameBufferSeries(buffer);
-    series->setXAxis(_xAxisAsIndex, _xMin, _xMax);
+    auto series = new FrameBufferSeries(xBuf, yBuf);
     curve->setSamples(series);
     _addCurve(curve);
 }
@@ -356,10 +358,12 @@ void PlotManager::_addCurve(QwtPlotCurve
     {
         // create a new plot widget
         plot = addPlotWidget();
+        plot->setDispChannels(QVector<const StreamChannel*>(1, _stream->channel(index)));
     }
     else
     {
         plot = plotWidgets[0];
+        plot->setDispChannels(_stream->allChannels());
     }
 
     // show the curve
@@ -481,10 +485,13 @@ void PlotManager::setXAxis(bool asIndex,
     _xAxisAsIndex = asIndex;
     _xMin = xMin;
     _xMax = xMax;
+
+    int ci = 0;
     for (auto curve : curves)
     {
         FrameBufferSeries* series = static_cast<FrameBufferSeries*>(curve->data());
-        series->setXAxis(asIndex, xMin, xMax);
+        series->setX(_stream->channel(ci)->xData());
+        ci++;
     }
     for (auto plot : plotWidgets)
     {
diff --git a/src/plotmanager.h b/src/plotmanager.h
--- a/src/plotmanager.h
+++ b/src/plotmanager.h
@@ -41,7 +41,7 @@ class PlotManager : public QObject
 
 public:
     explicit PlotManager(QWidget* plotArea, PlotMenu* menu,
-                         const Stream* stream = NULL,
+                         const Stream* stream = nullptr,
                          QObject *parent = 0);
     explicit PlotManager(QWidget* plotArea, PlotMenu* menu,
                          Snapshot* snapshot,
@@ -49,7 +49,7 @@ public:
     ~PlotManager();
     /// Add a new curve with title and buffer. A color is
     /// automatically chosen for curve.
-    void addCurve(QString title, const FrameBuffer* buffer);
+    void addCurve(QString title, const XFrameBuffer* xBuf, const FrameBuffer* yBuf);
     /// Removes curves from the end
     void removeCurves(unsigned number);
     /// Returns current number of curves known by plot manager
@@ -82,7 +82,7 @@ private:
     QList<QwtPlotCurve*> curves;
     QList<Plot*> plotWidgets;
     Plot* emptyPlot;  ///< for displaying when all channels are hidden
-    const Stream* _stream;       ///< attached stream, can be `NULL`
+    const Stream* _stream;       ///< attached stream, can be `nullptr`
     const ChannelInfoModel* infoModel;
     bool isDemoShown;
     bool _autoScaled;
diff --git a/src/snapshot.h b/src/snapshot.h
--- a/src/snapshot.h
+++ b/src/snapshot.h
@@ -28,6 +28,7 @@
 
 #include "channelinfomodel.h"
 #include "readonlybuffer.h"
+#include "indexbuffer.h"
 
 class SnapshotView;
 class MainWindow;
@@ -40,7 +41,8 @@ public:
     Snapshot(MainWindow* parent, QString name, ChannelInfoModel infoModel, bool saved = false);
     ~Snapshot();
 
-    // TODO: yData of snapshot shouldn't be public, preferable should be handled in constructor
+    // TODO: yData and xData of snapshot shouldn't be public, preferable should be handled in constructor
+    QVector<IndexBuffer*> xData;
     QVector<ReadOnlyBuffer*> yData;
     QAction* showAction();
     QAction* deleteAction();
diff --git a/src/snapshotmanager.cpp b/src/snapshotmanager.cpp
--- a/src/snapshotmanager.cpp
+++ b/src/snapshotmanager.cpp
@@ -71,6 +71,7 @@ Snapshot* SnapshotManager::makeSnapshot(
 
     for (unsigned ci = 0; ci < _stream->numChannels(); ci++)
     {
+        snapshot->xData.append(new IndexBuffer(_stream->numSamples()));
         snapshot->yData.append(new ReadOnlyBuffer(_stream->channel(ci)->yData()));
     }
 
@@ -194,6 +195,7 @@ void SnapshotManager::loadSnapshotFromFi
 
     for (unsigned ci = 0; ci < numOfChannels; ci++)
     {
+        snapshot->xData.append(new IndexBuffer(data[ci].size()));
         snapshot->yData.append(new ReadOnlyBuffer(data[ci].data(), data[ci].size()));
     }
 
diff --git a/src/stream.cpp b/src/stream.cpp
--- a/src/stream.cpp
+++ b/src/stream.cpp
@@ -20,6 +20,7 @@
 #include "stream.h"
 #include "ringbuffer.h"
 #include "indexbuffer.h"
+#include "linindexbuffer.h"
 
 Stream::Stream(unsigned nc, bool x, unsigned ns) :
     _infoModel(nc)
@@ -27,15 +28,20 @@ Stream::Stream(unsigned nc, bool x, unsi
     _numSamples = ns;
     _paused = false;
 
+    xAsIndex = true;
+    xMin = 0;
+    xMax = 1;
+
     // create xdata buffer
     _hasx = x;
     if (x)
     {
-        xData = new RingBuffer(ns);
+        // TODO: implement XRingBuffer (binary search)
+        Q_ASSERT(false);
     }
     else
     {
-        xData = new IndexBuffer(ns);
+        xData = makeXBuffer();
     }
 
     // create channels
@@ -81,6 +87,16 @@ StreamChannel* Stream::channel(unsigned 
     return const_cast<StreamChannel*>(static_cast<const Stream&>(*this).channel(index));
 }
 
+QVector<const StreamChannel*> Stream::allChannels() const
+{
+    QVector<const StreamChannel*> result(numChannels());
+    for (unsigned ci = 0; ci < numChannels(); ci++)
+    {
+        result[ci] = channel(ci);
+    }
+    return result;
+}
+
 const ChannelInfoModel* Stream::infoModel() const
 {
     return &_infoModel;
@@ -118,11 +134,12 @@ void Stream::setNumChannels(unsigned nc,
     {
         if (x)
         {
-            xData = new RingBuffer(_numSamples);
+            // TODO: implement XRingBuffer (binary search)
+            Q_ASSERT(false);
         }
         else
         {
-            xData = new IndexBuffer(_numSamples);
+            xData = makeXBuffer();
         }
 
         for (auto c : channels)
@@ -142,6 +159,18 @@ void Stream::setNumChannels(unsigned nc,
     Sink::setNumChannels(nc, x);
 }
 
+XFrameBuffer* Stream::makeXBuffer() const
+{
+    if (xAsIndex)
+    {
+        return new IndexBuffer(_numSamples);
+    }
+    else
+    {
+        return new LinIndexBuffer(_numSamples, xMin, xMax);
+    }
+}
+
 const SamplePack* Stream::applyGainOffset(const SamplePack& pack) const
 {
     Q_ASSERT(infoModel()->gainOrOffsetEn());
@@ -191,7 +220,9 @@ void Stream::feedIn(const SamplePack& pa
     unsigned ns = pack.numSamples();
     if (_hasx)
     {
-        static_cast<RingBuffer*>(xData)->addSamples(pack.xData(), ns);
+        // TODO: implement XRingBuffer (binary search)
+        Q_ASSERT(false);
+        // static_cast<RingBuffer*>(xData)->addSamples(pack.xData(), ns);
     }
 
     // modified pack that gain and offset is applied to
@@ -237,6 +268,24 @@ void Stream::setNumSamples(unsigned valu
     }
 }
 
+void Stream::setXAxis(bool asIndex, double min, double max)
+{
+    xAsIndex = asIndex;
+    xMin = min;
+    xMax = max;
+
+    // Note that x axis scaling is ignored when X is provided from source as data
+    // TODO: assert (UI options for x axis should be disabled)
+    if (!hasX())
+    {
+        xData = makeXBuffer();
+        for (auto c : channels)
+        {
+            c->setX(xData);
+        }
+    }
+}
+
 void Stream::saveSettings(QSettings* settings) const
 {
     _infoModel.saveSettings(settings);
diff --git a/src/stream.h b/src/stream.h
--- a/src/stream.h
+++ b/src/stream.h
@@ -48,16 +48,16 @@ public:
      * @param x has X data input
      * @param ns number of samples
      */
-    Stream(unsigned nc = 0, bool x = false, unsigned ns = 0);
+    Stream(unsigned nc = 1, bool x = false, unsigned ns = 2);
     ~Stream();
 
-    // implementations for `Source`
-    virtual bool hasX() const;
-    virtual unsigned numChannels() const;
+    bool hasX() const;
+    unsigned numChannels() const;
 
     unsigned numSamples() const;
     const StreamChannel* channel(unsigned index) const;
     StreamChannel* channel(unsigned index);
+    QVector<const StreamChannel*> allChannels() const;
     const ChannelInfoModel* infoModel() const;
     ChannelInfoModel* infoModel();
 
@@ -79,10 +79,13 @@ signals:
     void dataAdded(); ///< emitted when data added to channel man.
 
 public slots:
-    // TODO: these won't be public
-    // void setNumChannels(unsigned number);
+    /// Change number of samples (buffer size)
     void setNumSamples(unsigned value);
 
+    /// Change X axis style
+    /// @note Ignored when X is provided by source (hasX == true)
+    void setXAxis(bool asIndex, double min, double max);
+
     /// When paused data feed is ignored
     void pause(bool paused);
 
@@ -94,11 +97,14 @@ private:
     bool _paused;
 
     bool _hasx;
-    ResizableBuffer* xData;
+    XFrameBuffer* xData;
     QList<StreamChannel*> channels;
 
     ChannelInfoModel _infoModel;
 
+    bool xAsIndex;
+    double xMin, xMax;
+
     /**
      * Applies gain and offset to given pack.
      *
@@ -111,6 +117,9 @@ private:
      * @return modified data
      */
     const SamplePack* applyGainOffset(const SamplePack& pack) const;
+
+    /// Returns a new virtual X buffer for settings
+    XFrameBuffer* makeXBuffer() const;
 };
 
 
diff --git a/src/streamchannel.cpp b/src/streamchannel.cpp
--- a/src/streamchannel.cpp
+++ b/src/streamchannel.cpp
@@ -17,9 +17,10 @@
   along with serialplot.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include <limits>
 #include "streamchannel.h"
 
-StreamChannel::StreamChannel(unsigned i, const FrameBuffer* x,
+StreamChannel::StreamChannel(unsigned i, const XFrameBuffer* x,
               FrameBuffer* y, ChannelInfoModel* info)
 {
     _index = i;
@@ -37,8 +38,37 @@ unsigned StreamChannel::index() const {r
 QString StreamChannel::name() const {return _info->name(_index);};
 QColor StreamChannel::color() const {return _info->color(_index);};
 bool StreamChannel::visible() const {return _info->isVisible(_index);};
-const FrameBuffer* StreamChannel::xData() const {return _x;}
+const XFrameBuffer* StreamChannel::xData() const {return _x;}
 const FrameBuffer* StreamChannel::yData() const {return _y;}
 FrameBuffer* StreamChannel::yData() {return _y;}
 const ChannelInfoModel* StreamChannel::info() const {return _info;}
-void StreamChannel::setX(const FrameBuffer* x) {_x = x;};
+void StreamChannel::setX(const XFrameBuffer* x) {_x = x;};
+
+double StreamChannel::findValue(double x) const
+{
+    int index = _x->findIndex(x);
+    Q_ASSERT(index < (int) _x->size());
+
+    if (index >= 0)
+    {
+        // can't do estimation for last sample
+        if (index == (int) _x->size() - 1)
+        {
+            return _y->sample(index);
+        }
+        else
+        {
+            // calculate middle of the line
+            double prev_x = _x->sample(index);
+            double next_x = _x->sample(index+1);
+            double ratio = (x - prev_x) / (next_x - prev_x);
+            double prev_y = _y->sample(index);
+            double next_y = _y->sample(index+1);
+            return ratio * (next_y - prev_y) + prev_y;
+        }
+    }
+    else
+    {
+        return std::numeric_limits<double>::quiet_NaN();
+    }
+}
diff --git a/src/streamchannel.h b/src/streamchannel.h
--- a/src/streamchannel.h
+++ b/src/streamchannel.h
@@ -35,7 +35,7 @@ public:
      * @param info channel info model
      */
     StreamChannel(unsigned i,
-                  const FrameBuffer* x,
+                  const XFrameBuffer* x,
                   FrameBuffer* y,
                   ChannelInfoModel* info);
     ~StreamChannel();
@@ -44,15 +44,23 @@ public:
     QString name() const;
     QColor color() const;
     bool visible() const;
-    const FrameBuffer* xData() const;
+    const XFrameBuffer* xData() const;
     FrameBuffer* yData();
     const FrameBuffer* yData() const;
     const ChannelInfoModel* info() const;
-    void setX(const FrameBuffer* x);
+    void setX(const XFrameBuffer* x);
+
+    /**
+     * Returns sample value for `x`.
+     *
+     * If `x` is out of range `NaN` is returned. A calculated (linear)
+     * value is returned when `x` is in between two data points.
+     */
+    double findValue(double x) const;
 
 private:
     unsigned _index;
-    const FrameBuffer* _x;
+    const XFrameBuffer* _x;
     FrameBuffer* _y;
     ChannelInfoModel* _info;
 };
diff --git a/src/zoomer.cpp b/src/zoomer.cpp
--- a/src/zoomer.cpp
+++ b/src/zoomer.cpp
@@ -1,5 +1,5 @@
 /*
-  Copyright © 2017 Hasan Yavuz Özderya
+  Copyright © 2018 Hasan Yavuz Özderya
 
   This file is part of serialplot.
 
@@ -19,9 +19,12 @@
 
 #include "zoomer.h"
 #include <qwt_plot.h>
-#include <QtDebug>
+#include <QPen>
+#include <QMouseEvent>
+#include <QtMath>
 
-#include <QMouseEvent>
+static const int VALUE_POINT_DIAM = 4;
+static const int VALUE_TEXT_MARGIN = VALUE_POINT_DIAM + 2;
 
 Zoomer::Zoomer(QWidget* widget, bool doReplot) :
     ScrollZoomer(widget)
@@ -59,6 +62,11 @@ void Zoomer::zoom( const QRectF & rect)
     ScrollZoomer::zoom(rect);
 }
 
+void Zoomer::setDispChannels(QVector<const StreamChannel*> channels)
+{
+    dispChannels = channels;
+}
+
 QwtText Zoomer::trackerTextF(const QPointF& pos) const
 {
     QwtText b = ScrollZoomer::trackerTextF(pos);
@@ -102,6 +110,96 @@ QRegion Zoomer::rubberBandMask() const
     return QRegion(r);
 }
 
+void Zoomer::drawTracker(QPainter* painter) const
+{
+    if (isActive())
+    {
+        QwtPlotZoomer::drawTracker(painter);
+    }
+    else if (dispChannels.length())
+    {
+        drawValues(painter);
+    }
+}
+
+void Zoomer::drawValues(QPainter* painter) const
+{
+    painter->save();
+
+    double x = invTransform(trackerPosition()).x();
+    auto values = findValues(x);
+
+    // draw vertical line
+    auto linePen = rubberBandPen();
+    linePen.setStyle(Qt::DotLine);
+    painter->setPen(linePen);
+    const QRect pRect = pickArea().boundingRect().toRect();
+    int px = trackerPosition().x();
+    painter->drawLine(px, pRect.top(), px, pRect.bottom());
+
+    // draw sample values
+    for (int ci = 0; ci < values.size(); ci++)
+    {
+        if (!dispChannels[ci]->visible()) continue;
+
+        double val = values[ci];
+        if (!std::isnan(val))
+        {
+            auto p = transform(QPointF(x, val));
+
+            painter->setBrush(dispChannels[ci]->color());
+            painter->setPen(Qt::NoPen);
+            painter->drawEllipse(p, VALUE_POINT_DIAM, VALUE_POINT_DIAM);
+
+            painter->setPen(rubberBandPen());
+            // We give a very small (1x1) rectangle but disable clipping
+            painter->drawText(QRectF(p.x() + VALUE_TEXT_MARGIN, p.y(), 1, 1),
+                              Qt::AlignVCenter | Qt::TextDontClip,
+                              QString("%1").arg(val));
+        }
+    }
+
+    painter->restore();
+}
+
+QVector<double> Zoomer::findValues(double x) const
+{
+    unsigned nc = dispChannels.length();
+    QVector<double> result(nc);
+    for (unsigned ci = 0; ci < nc; ci++)
+    {
+        if (dispChannels[ci]->visible())
+        {
+            result[ci] = dispChannels[ci]->findValue(x);
+        }
+
+    }
+    return result;
+}
+
+QRect Zoomer::trackerRect(const QFont& font) const
+{
+    if (isActive())
+    {
+        return QwtPlotZoomer::trackerRect(font);
+    }
+    else
+    {
+        return valueTrackerRect(font);
+    }
+}
+
+QRect Zoomer::valueTrackerRect(const QFont& font) const
+{
+    // TODO: consider using actual tracker values for width calculation
+    const int textWidth = qCeil(QwtText("-8.8888888").textSize(font).width());
+    const int width = textWidth + VALUE_POINT_DIAM + VALUE_TEXT_MARGIN;
+    const int x = trackerPosition().x() - VALUE_POINT_DIAM;
+    const auto pickRect = pickArea().boundingRect();
+
+    return QRect(x, pickRect.y(), width, pickRect.height());
+}
+
 void Zoomer::widgetMousePressEvent(QMouseEvent* mouseEvent)
 {
     if (mouseEvent->modifiers() & Qt::ControlModifier)
diff --git a/src/zoomer.h b/src/zoomer.h
--- a/src/zoomer.h
+++ b/src/zoomer.h
@@ -1,5 +1,5 @@
 /*
-  Copyright © 2017 Hasan Yavuz Özderya
+  Copyright © 2018 Hasan Yavuz Özderya
 
   This file is part of serialplot.
 
@@ -20,25 +20,35 @@
 #ifndef ZOOMER_H
 #define ZOOMER_H
 
-#include <scrollzoomer.h>
+#include <QVector>
+#include <QRect>
+
+#include "scrollzoomer.h"
+#include "streamchannel.h"
 
 class Zoomer : public ScrollZoomer
 {
     Q_OBJECT
 
 public:
-    Zoomer(QWidget *, bool doReplot=true);
+    Zoomer(QWidget*, bool doReplot=true);
     void zoom(int up);
-    void zoom( const QRectF & );
+    void zoom(const QRectF&);
+    /// Set displayed channels for value tracking (can be null)
+    void setDispChannels(QVector<const StreamChannel*> channels);
 
 signals:
     void unzoomed();
 
 protected:
     /// Re-implemented to display selection size in the tracker text.
-    QwtText trackerTextF(const QPointF &pos) const;
+    QwtText trackerTextF(const QPointF &pos) const override;
+    /// Re-implemented for sample value tracker
+    QRect trackerRect(const QFont&) const override;
     /// Re-implemented for alpha background
-    void drawRubberBand(QPainter* painter) const;
+    void drawRubberBand(QPainter* painter) const override;
+    /// Re-implemented to draw sample values
+    void drawTracker(QPainter* painter) const override;
     /// Re-implemented for alpha background (masking is basically disabled)
     QRegion rubberBandMask() const;
     /// Overloaded for panning
@@ -51,6 +61,15 @@ protected:
 private:
     bool is_panning;
     QPointF pan_point;
+    /// displayed channels for value tracking
+    QVector<const StreamChannel*> dispChannels;
+
+    /// Draw sample values
+    void drawValues(QPainter* painter) const;
+    /// Find sample values for given X value
+    QVector<double> findValues(double x) const;
+    /// Returns trackerRect for value tracker
+    QRect valueTrackerRect(const QFont& font) const;
 };
 
 #endif // ZOOMER_H
diff --git a/tests/test.cpp b/tests/test.cpp
--- a/tests/test.cpp
+++ b/tests/test.cpp
@@ -221,6 +221,14 @@ TEST_CASE("LinIndexBuffer", "[memory, bu
 
     REQUIRE(buf.sample(0) == -5.0);
     REQUIRE(buf.sample(19) == 5.0);
+
+    buf.resize(10);
+    buf.setLimits({0., 3.});
+    REQUIRE(buf.findIndex(0.01) == 0);
+    REQUIRE(buf.findIndex(1.51) == 4);
+    REQUIRE(buf.findIndex(2.99) == 8);
+    REQUIRE(buf.findIndex(3.01) == XFrameBuffer::OUT_OF_RANGE);
+    REQUIRE(buf.findIndex(-0.01) == XFrameBuffer::OUT_OF_RANGE);
 }
 
 TEST_CASE("RingBuffer sizing", "[memory, buffer]")
diff --git a/tests/test_stream.cpp b/tests/test_stream.cpp
--- a/tests/test_stream.cpp
+++ b/tests/test_stream.cpp
@@ -27,17 +27,17 @@ TEST_CASE("construction of stream with d
     // default values are an empty stream with no channels
     Stream s;
 
-    REQUIRE(s.numChannels() == 0);
+    REQUIRE(s.numChannels() == 1);
     REQUIRE(!s.hasX());
-    REQUIRE(s.numSamples() == 0);
+    REQUIRE(s.numSamples() == 2);
 }
 
 TEST_CASE("construction of stream with parameters", "[memory, stream]")
 {
-    Stream s(4, true, 100);
+    Stream s(4, false, 100);
 
     REQUIRE(s.numChannels() == 4);
-    REQUIRE(s.hasX());
+    REQUIRE(!s.hasX());
     REQUIRE(s.numSamples() == 100);
 
     for (unsigned i = 0; i < 4; i++)
@@ -64,6 +64,8 @@ TEST_CASE("changing stream number of cha
         REQUIRE(c->index() == i);
     }
 
+// TODO: enable test when `Stream` supports X channel
+#if 0
     // increase nc value, add X
     so._setNumChannels(5, true);
 
@@ -76,6 +78,7 @@ TEST_CASE("changing stream number of cha
         REQUIRE(c != NULL);
         REQUIRE(c->index() == i);
     }
+#endif
 
     // reduce nc value, remove X
     so._setNumChannels(1, false);
@@ -127,9 +130,11 @@ TEST_CASE("adding data to a stream with 
     }
 }
 
+// TODO: enable test when `Stream` supports X channel
+#if 0
 TEST_CASE("adding data to a stream with X", "[memory, stream, data, sink]")
 {
-    Stream s(3, true, 10);
+    Stream s(3, false, 10);
 
     // prepare data
     SamplePack pack(5, 3, true);
@@ -147,7 +152,7 @@ TEST_CASE("adding data to a stream with 
     }
 
     TestSource so(3, true);
-    so.connectSink(&s);
+    REQUIRE_THROWS(so.connectSink(&s));
 
     // test
     so._feed(pack);
@@ -178,6 +183,7 @@ TEST_CASE("adding data to a stream with 
         REQUIRE(x->sample(i) == (i-5)+10);
     }
 }
+#endif
 
 TEST_CASE("paused stream shouldn't store data", "[memory, stream, pause]")
 {