code: def _reset_bbox(self):
          
          scale_x, scale_y = self.get_scale_xy()
          pan_x, pan_y = self.get_pan(coord='data')[:2]
          win_wd, win_ht = self.get_window_size()
          # NOTE: need to set at least a minimum 1-pixel dimension on
          # the window or we get a scale calculation exception. See github
          # issue 431
          win_wd, win_ht = max(1, win_wd), max(1, win_ht)
  
          self._calc_bg_dimensions(scale_x, scale_y,
                                   pan_x, pan_y, win_wd, win_ht)
docs: This function should only be called internally.  It resets
          the viewers bounding box based on changes to pan or scale.