ok, I removed the QString::number(myPlot.ValueAt(pos.x(), pos.y())

Qt Code: 
  1. class PlotPicker : public QwtPlotPicker
  2. {
  3. public:
  4.  
  5.     PlotPicker(QwtPlot::Axis xAxis, QwtPlot::Axis yAxis, QwtPicker::RubberBand rb, QwtPicker::DisplayMode dm, QwtPlotCanvas* canvas):
  6.             QwtPlotPicker( xAxis,  yAxis, rb , dm , canvas)
  7.     {
  8.  
  9.         setStateMachine(new QwtPickerPolygonMachine());
  10.         setRubberBandPen(QColor(Qt::green));
  11.         //d_picker->setRubberBand(QwtPicker::PolygonRubberBand);
  12.         //setTrackerPen(QColor(Qt::yellow));
  13.         //setTrackerFont(f);
  14.     }
  15.  
  16.     QwtText trackerText (const QwtDoublePoint & pos) const;
  17.  
  18. };
  19.  
  20. QwtText PlotPicker::trackerText (const QwtDoublePoint & pos) const
  21. {
  22.     QwtText text("(" + QString::number(pos.x()) + "...," + QString::number(pos.y()) + ") " );
  23.     QColor bgColor(Qt::blue);
  24.     bgColor.setAlpha(160);
  25.     text.setBackgroundBrush(QBrush(bgColor));
  26.     return text;
  27. }
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but the function trackerText is never actually called. I changed several parameters (colour, text, etc) but nothing happens!