QwtPlotZoomer zooming off the plot canvas

I am using QwtPlotZoomer with a few graphs in my application but I am having a few problems. I do not want to allow the user to zoom off of the original canvas that is shown with the Plot on it, ie; zooming in to negatives.

Code:

---

class YourZoomer: public QwtPlotZoomer
{
    ....
 
    virtual void zoom( const QRectF &rect )
    {
        QwtPlotZoomer::zoom( rect & zoomBase() );
    }
};

---

Uwe