See db.Database.

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