message: [error handling] 'Time Comparison' query returns no data
diff: @@ -1130,6 +1130,7 @@ class NVD3TimeSeriesViz(NVD3Viz):
  query_object['to_dttm'] -= delta
  df2 = self.get_df_payload(query_object).get('df')
  + if df2 is not None:
  df2[DTTM_ALIAS] += delta
  df2 = self.process_data(df2)
  self.extra_chart_data = self.to_series(
  