code: public static DataSet last(DataSet data, int n) {
      return subset(data, data.size() - n, n);
    }
docs: Create sub-set with the last <code>n</code> rows.
  @param data Source data set.
  @param n Number of rows.
  @return A new data set containing the last
  <code>n</code> rows in the source data set.
  @throws InvalidOperationException if the given data
  set is null or if the value of <code>n</code> is invalid.