The panner only grabs the content of the canvas to a pixmap that is moved around while panning. The reason behind this implementation is, that rescaling might be too slow ( depending on content of your plot ) for mouse movements. Guess you might also have some effects because of the jumping scales.

If you want immediate updates of the plot simply implement an event filter for the plot canvas, where you map the mouse move events to scale changes ( like the panner does ).

Uwe