code: function getSortFn (dataType, sortType, index) {
      let retFunc;
      switch (dataType) {
      case MeasureSubtype.CONTINUOUS:
      case DimensionSubtype.TEMPORAL:
          if (sortType === 'desc') {
              retFunc = (a, b) => b[index] - a[index];
          } else {
              retFunc = (a, b) => a[index] - b[index];
          }
          break;
      default:
          retFunc = (a, b) => {
              const a1 = `${a[index]}`;
              const b1 = `${b[index]}`;
              if (a1 < b1) {
                  return sortType === 'desc' ? 1 : -1;
              }
              if (a1 > b1) {
                  return sortType === 'desc' ? -1 : 1;
              }
              return 0;
          };
      }
      return retFunc;
  }
docs: Generates the sorting functions to sort the data of a DataModel instance
  according to the input data type.
  
  @param {string} dataType - The data type e.g. 'measure', 'datetime' etc.
  @param {string} sortType - The sorting order i.e. 'asc' or 'desc'.
  @param {integer} index - The index of the data which will be sorted.
  @return {Function} Returns the the sorting function.