diff --git a/mainwindow.h b/mainwindow.h
--- a/mainwindow.h
+++ b/mainwindow.h
@@ -33,6 +33,7 @@
 #include <QtGlobal>
 #include <qwt_plot_curve.h>
 #include <qwt_plot_textlabel.h>
+#include "zoomer.h"
 
 #include "portcontrol.h"
 #include "ui_about_dialog.h"
@@ -85,6 +86,8 @@ private:
     // `data` contains i th channels data
     void addChannelData(unsigned int channel, DataArray data);
 
+    Zoomer* zoomer;
+
     NumberFormat numberFormat;
     unsigned int sampleSize; // number of bytes in the selected number format
     double (MainWindow::*readSample)();