def ms_pan(self, viewer, event, data_x, data_y):
        """A 'drag' or proportional pan, where the image is panned by
        'dragging the canvas' up or down.  The amount of the pan is
        proportionate to the length of the drag.
        """
        if not self.canpan:
            return True

        x, y = viewer.get_last_win_xy()
        if event.state == 'move':
            data_x, data_y = self.get_new_pan(viewer, x, y,
                                              ptype=self._pantype)
            viewer.panset_xy(data_x, data_y)

        elif event.state == 'down':
            self.pan_set_origin(viewer, x, y, data_x, data_y)
            self.pan_start(viewer, ptype=2)

        else:
            self.pan_stop(viewer)
        return True
A 'drag' or proportional pan, where the image is panned by
        'dragging the canvas' up or down.  The amount of the pan is
        proportionate to the length of the drag.