message: [line] improve  feature
  Require Since and Until parameter as they are needed to time shift
diff: @@ -1073,6 +1073,11 @@ class NVD3TimeSeriesViz(NVD3Viz):
  delta = utils.parse_human_timedelta(time_compare)
  query_object['inner_from_dttm'] = query_object['from_dttm']
  query_object['inner_to_dttm'] = query_object['to_dttm']
  +
  + if not query_object['from_dttm'] or not query_object['to_dttm']:
  + raise Exception(_(
  + '`Since` and `Until` time bounds should be specified '
  + 'when using the `Time Shift` feature.'))
  query_object['from_dttm'] -= delta
  query_object['to_dttm'] -= delta
  