code: public GroupStatListResult getGroupStatistic(String start, int duration)
              throws APIConnectionException, APIRequestException {
          return _reportClient.getGroupStatistic(start, duration);
      }
docs: Get group statistic, time unit only supports DAY now.
  @param start Format: yyyy-MM-dd
  @param duration duration must between 0 and 60
  @return {@link GroupStatListResult}
  @throws APIConnectionException connect exception
  @throws APIRequestException request exception