code: def _init_plot_handles(self):
          
          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
docs: Find all requested plotting handles and cache them along
          with the IDs of the models the callbacks will be attached to.