code: def kp_pan_zoom_save(self, viewer, event, data_x, data_y, msg=True):
          
          reg = 1
          scale = viewer.get_scale_xy()
          self._save[(viewer, 'scale', reg)] = scale
          if msg:
              viewer.onscreen_message("Saved current scale", delay=0.5)
          return True
docs: Save the current viewer scale for future use with
          kp_pan_zoom_set().