How customize NavigationToolbar to change x, y mouse over in Qt4 backend


What is the right way to change the x,y text that appears at the right end of the NavigationToolbar? I like it but I would like to change the x value by a function call like:

newx = (time(x))

I am using Qt4 backend. I would like to init a copy of the NavigationToolbar and made the necessary changes. The problem is that I can not find the source code for the NavigationToolbar & qt4.

I am also interesting in deleting the subplot configuration tool icon (not used) but don't know what to do.

Any help is appreciated.