docs: def close(self):
          """This method closes the canvas and writes
          contents to the associated file.
          Calling this procedure is optional, because
          Pychart calls this procedure for every open canvas on normal exit."""
          for i in range(0, len(active_canvases)):
              if active_canvases[i] == self:
                  del active_canvases[i]
                  return