code: public String formatInterval(Interval interval, DataColumn column) {
  
          // Raw values
          if (column == null || column.getColumnGroup() == null) {
              return interval.getName();
          }
          // Date interval
          String type = interval.getType();
          if (StringUtils.isBlank(type)) type = column.getIntervalType();
          if (StringUtils.isBlank(type)) type = column.getColumnGroup().getIntervalSize();
          DateIntervalType intervalType = DateIntervalType.getByName(type);
          if (intervalType != null) {
              ColumnSettings columnSettings = displayerSettings.getColumnSettings(column.getId());
              String pattern = columnSettings != null ? columnSettings.getValuePattern() : ColumnSettings.getDatePattern(intervalType);
              String expression = columnSettings != null ? columnSettings.getValueExpression() : null;
  
              if (pattern == null) {
                  pattern = ColumnSettings.getDatePattern(intervalType);
              }
              if (expression == null && column.getColumnGroup().getStrategy().equals(GroupStrategy.FIXED)) {
                  expression = ColumnSettings.getFixedExpression(intervalType);
              }
  
              return formatDate(intervalType,
                      column.getColumnGroup().getStrategy(),
                      interval.getName(), pattern, expression);
          }
          // Label interval
          ColumnSettings columnSettings = displayerSettings.getColumnSettings(column);
          String expression = columnSettings.getValueExpression();
          if (StringUtils.isBlank(expression)) return interval.getName();
          return getEvaluator().evalExpression(interval.getName(), expression);
      }
docs: DATA FORMATTING