Fetches the latest graph series for a client-label from the DB.

def ReadMostRecentClientGraphSeries(
      self,
      client_label,
      report_type,
      cursor=None):
    """Fetches the latest graph series for a client-label from the DB."""
    query = """
      SELECT graph_series
      FROM client_report_graphs
      WHERE client_label = %s AND report_type = %s
      ORDER BY timestamp DESC
      LIMIT 1
    """
    args = [client_label, report_type.SerializeToDataStore()]
    cursor.execute(query, args)
    result = cursor.fetchone()
    if result is None:
      return None
    else:
      return rdf_stats.ClientGraphSeries.FromSerializedString(result[0])