about the image callback in gui

hi, I am trying to develop the gui which has two axes. first axes shows the image and second axes shows the projection of that image on one axis and its fitting.

Here is what I want to do but I am struggling:

I can use tool bar to zoom in the image on first axes. now I have zoomed image on my first axes. I would like the projection and fitting to be done according to the image I am seeing on the first axes accordingly. So I need some sort of callback in my script.

Is there anybody who can help me out?

thanks