Cleans up references to the plot after the plot has been
        deleted. Traverses through all plots cleaning up Callbacks and
        Stream subscribers.

def cleanup(self):
        """
        Cleans up references to the plot after the plot has been
        deleted. Traverses through all plots cleaning up Callbacks and
        Stream subscribers.
        """
        plots = self.traverse(lambda x: x, [BokehPlot])
        for plot in plots:
            if not isinstance(plot, (GenericCompositePlot, GenericElementPlot, GenericOverlayPlot)):
                continue
            streams = list(plot.streams)
            plot.streams = []
            plot._document = None

            if plot.subplots:
                plot.subplots.clear()

            if isinstance(plot, GenericElementPlot):
                for callback in plot.callbacks:
                    streams += callback.streams
                    callback.cleanup()

            for stream in set(streams):
                stream._subscribers = [
                    (p, subscriber) for p, subscriber in stream._subscribers
                    if get_method_owner(subscriber) not in plots
                ]

        if self.comm and self.root is self.handles.get('plot'):
            self.comm.close()