code: def aggregations(self):
          
  
          prev_month_start = get_prev_month(self.end, self.query.interval_)
          self.query.since(prev_month_start)
          self.query.get_terms("author_name")
          return self.query.get_list(dataframe=True)
docs: Override parent method. Obtain list of the terms and their corresponding
          values using "terms" aggregations for the previous time period.
  
          :returns: a data frame containing terms and their corresponding values