Cleans up references to the plot on the attached Stream
        subscribers.

def cleanup(self):
        """
        Cleans up references to the plot on the attached Stream
        subscribers.
        """
        plots = self.traverse(lambda x: x, [Plot])
        for plot in plots:
            if not isinstance(plot, (GenericCompositePlot, GenericElementPlot, GenericOverlayPlot)):
                continue
            for stream in set(plot.streams):
                stream._subscribers = [
                    (p, subscriber) for p, subscriber in stream._subscribers
                    if util.get_method_owner(subscriber) not in plots]
        if self.comm:
            self.comm.close()