QWT Zoomer Problems

 

Using the Bode example I was able to implement the zoomer functionality in a few plots that I have created.

The first plots x-axis begins at zero and I have noticed that when zooming in it is possible to zoom into negative coordinates which I would like to disable. I am thinking their is either two ways to do it which would be to not allow the mouse to go off the plot area or when the user zooms in the minimum value for the x-axis is set back to 0. Any help or suggestions would be appreciated.