Find all requested plotting handles and cache them along
        with the IDs of the models the callbacks will be attached to.

def _init_plot_handles(self):
        """
        Find all requested plotting handles and cache them along
        with the IDs of the models the callbacks will be attached to.
        """
        plots = [self.plot]
        if self.plot.subplots:
            plots += list(self.plot.subplots.values())

        handles = {}
        for plot in plots:
            for k, v in plot.handles.items():
                handles[k] = v
        self.plot_handles = handles

        requested = {}
        for h in self.models+self.extra_models:
            if h in self.plot_handles:
                requested[h] = handles[h]
            elif h in self.extra_models:
                print("Warning %s could not find the %s model. "
                      "The corresponding stream may not work."
                      % (type(self).__name__, h))
        self.handle_ids.update(self._get_stream_handle_ids(requested))

        return requested