Something like this ?

Qt Code: 
  1. class DistancePicker : public QwtPlotPicker
  2. {
  3. public:
  4.    DistancePicker( QwtPlotCanvas*);
  5.  
  6. protected:
  7.    virtual void drawRubberBand(QPainter*) const;
  8.    virtual QwtText trackerText(const QwtDoublePoint& pos) const;
  9. };
  10.  
  11. DistancePicker::DistancePicker(QwtPlotCanvas* canvas):
  12.    QwtPlotPicker(canvas),  
  13. {    
  14.    /*
  15.       We don't have a picker for a line, but the rectangle
  16.       selection is also selection of 2 points, so all we
  17.       have to do is to change the rubberband.
  18.     */
  19.    setSelectionFlags(QwtPicker::RectSelection);
  20.    setRubberBand(PolygonRubberBand);
  21.    setTrackerMode(QwtPicker::ActiveOnly);
  22. }  
  23.  
  24. void DistancePicker::drawRubberBand(QPainter *painter) const
  25. {
  26.    painter->drawPolygon(selection());
  27. }  
  28.  
  29. QwtText DistancePicker::trackerText(const QwtDoublePoint &pos) const
  30. {
  31.    QwtText text( " selection() as distance string " );
  32.  
  33.    QColor bg(Qt::white);
  34.    bg.setAlpha(180);
  35.    text.setBackgroundBrush(QBrush(bg));
  36.    return text;
  37. }
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