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;
 };