Plot moves while using the "Zoom to rectangle" button

I have exactly the same problem on a program that I'm writing.
I attach a figure to show what I see.
I run python 2.6.2, Matplotlib, OpenSUSE 11.2 64 bit.

I thought that there was a problem with how I wrote the code since I'm a real beginner, but I'm "happy" to see that the problem is different.

My program has labels and entries, no comboboxes...

Does anyone know how to do with this?