got it I had to write my own subclass of QwtPlotPicker and overriding the "trackerText" method:

Qt Code: 
  1. class MyPicker : public QwtPlotPicker
  2.     {
  3.         Q_OBJECT
  4.  
  5.     public:
  6.         MyPicker(int xAxis, int yAxis, int selectionFlags, RubberBand rubberBand, DisplayMode trackerMode, QwtPlotCanvas* canvas);
  7.  
  8.     signals:
  9.         void mouseMoved(const QPoint& pos) const;    
  10.  
  11.     protected:
  12.             virtual QwtText trackerText (const QwtDoublePoint & pos) const;
  13.  
  14.     };
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Qt Code: 
  1. MyPicker::MyPicker(int xAxis, int yAxis, int selectionFlags, RubberBand rubberBand, DisplayMode trackerMode, QwtPlotCanvas* canvas)
  2.             : QwtPlotPicker(xAxis, yAxis, selectionFlags, rubberBand, trackerMode, canvas)
  3.         {}
  4.  
  5. QwtText MyPicker::trackerText (const QwtDoublePoint & pos) const
  6.     {
  7.         const QPoint point = pos.toPoint();
  8.         emit mouseMoved(point);
  9.         return QwtText(QString::number(point.x()) + ", " + QString::number(point.y()));
  10.     }
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now I can react to the "mouseMoved"-signal