Generate a histogram object and store in the specified attribute.

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)