def start_pan(self, x, y, button):
self._pan_start = cbook.Bunch(<EOL>lim           = self.viewLim.frozen(),<EOL>trans         = self.transData.frozen(),<EOL>trans_inverse = self.transData.inverted().frozen(),<EOL>bbox          = self.bbox.frozen(),<EOL>x             = x,<EOL>y             = y<EOL>)<EOL>
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.