code: def ms_panset(self, viewer, event, data_x, data_y,
                    msg=True):
          
          if self.canpan and (event.state == 'down'):
              self._panset(viewer, data_x, data_y, msg=msg)
          return True
docs: An interactive way to set the pan position.  The location
          (data_x, data_y) will be centered in the window.