'Set the child axes which are currently grabbing the mouse events.
Usually called by the widgets themselves.
It is an error to call this if the mouse is already grabbed by
another axes.'

def grab_mouse(self, ax):

    if (self.mouse_grabber not in (None, ax)):
        raise RuntimeError(u'Another    Axes    already    grabs    mouse    input')
    self.mouse_grabber = ax
