docs: def ReadMostRecentClientGraphSeries(self, client_label,
                                        report_type
                                       ):
      """See db.Database."""
      series_with_timestamps = self.ReadAllClientGraphSeries(
          client_label, report_type)
      if not series_with_timestamps:
        return None
      _, latest_series = list(sorted(iteritems(series_with_timestamps)))[-1]
      return latest_series