docs: def Save(self, token=None):
      """Generate a histogram object and store in the specified attribute."""
      graph_series_by_label = {}
      for active_time in self.active_days:
        for label in self.categories[active_time]:
          graphs_for_label = graph_series_by_label.setdefault(
              label, rdf_stats.ClientGraphSeries(report_type=self._report_type))
          graph = rdf_stats.Graph(title="%s day actives for %s label" %
                                  (active_time, label))
          for k, v in sorted(iteritems(self.categories[active_time][label])):
            graph.Append(label=k, y_value=v)
          graphs_for_label.graphs.Append(graph)
  
      for label, graph_series in iteritems(graph_series_by_label):
        client_report_utils.WriteGraphSeries(graph_series, label, token=token)