code: def _construct_callbacks(self):
          
          cb_classes = set()
          registry = list(Stream.registry.items())
          callbacks = Stream._callbacks['bokeh']
          for source in self.link_sources:
              streams = [
                  s for src, streams in registry for s in streams
                  if src is source or (src._plot_id is not None and
                                       src._plot_id == source._plot_id)]
              cb_classes |= {(callbacks[type(stream)], stream) for stream in streams
                             if type(stream) in callbacks and stream.linked
                             and stream.source is not None}
          cbs = []
          sorted_cbs = sorted(cb_classes, key=lambda x: id(x[0]))
          for cb, group in groupby(sorted_cbs, lambda x: x[0]):
              cb_streams = [s for _, s in group]
              cbs.append(cb(self, cb_streams, source))
          return cbs
docs: Initializes any callbacks for streams which have defined
          the plotted object as a source.