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.

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