code: @Override
      protected Map<ConcurrentUserAggregationDiscriminator, SortedSet<ConcurrentUserAggregation>>
              createColumnDiscriminatorMap(ConcurrentUserReportForm form) {
          return getDefaultGroupedColumnDiscriminatorMap(form);
      }
docs: Create a map of the report column discriminators based on the submitted form to collate the
  aggregation data into each column of a report. The map entries are a time-ordered sorted set
  of aggregation data points.
  
  @param form Form submitted by the user
  @return Map of report column discriminators to sorted set of time-based aggregation data