QWTPlot Zoom: cannot zoom negative value

Hi all,

I followed the bode example to create a simple XY curve plot with zooming capability. Weird thing is that unlike the bode program, mine does not zoom on negative scale as the bode does. For example, the curve has the Y (vertical axiz) values ranging from -100 to +100. This displays fine when the program pops up. But when users try to zoom say for Y from -50 to +50, it only shows for Y from 0 to +50!

Could anybody give me a hint how to solve this problem? I use the exact structure shown in the bode example.

Sincerely,