diff --git a/src/zoomer.h b/src/zoomer.h
--- a/src/zoomer.h
+++ b/src/zoomer.h
@@ -1,5 +1,5 @@
 /*
-  Copyright © 2016 Hasan Yavuz Özderya
+  Copyright © 2017 Hasan Yavuz Özderya
 
   This file is part of serialplot.
 
@@ -33,6 +33,10 @@ public:
 
 signals:
     void unzoomed();
+
+protected:
+    /// Re-implemented to display selection size in the tracker text.
+    QwtText trackerTextF(const QPointF &pos) const;
 };
 
 #endif // ZOOMER_H