'Called when a pan operation has started.
*x*, *y* are the mouse coordinates in display coords.
button is the mouse button number:
* 1: LEFT
* 2: MIDDLE
* 3: RIGHT
.. note::
Intended to be overridden by new projection types.'

def start_pan(self, x, y, button):

    self._pan_start = cbook.Bunch(lim=self.viewLim.frozen(), trans=self.transData.frozen(), trans_inverse=self.transData.inverted().frozen(), bbox=self.bbox.frozen(), x=x, y=y)
