code: def show_pan_mark(viewer, tf, color='red'):
      
      tag = '_$pan_mark'
      radius = 10
  
      canvas = viewer.get_private_canvas()
      try:
          mark = canvas.get_object_by_tag(tag)
          if not tf:
              canvas.delete_object_by_tag(tag)
          else:
              mark.color = color
  
      except KeyError:
          if tf:
              Point = canvas.get_draw_class('point')
              canvas.add(Point(0, 0, radius, style='plus', color=color,
                               coord='cartesian'),
                         tag=tag, redraw=False)
  
      canvas.update_canvas(whence=3)
docs: Show a mark in the pan position (center of window).
  
      Parameters
      ----------
      viewer : an ImageView subclass instance
          If True, show the color bar; else remove it if present.
  
      tf : bool
          If True, show the mark; else remove it if present.
  
      color : str
          Color of the mark; default is 'red'.