@Test
public void testSortIndex() {

    DataFrame df = DataFrame.newFrame("a", "b").foldByRow(
            1, "x",
            2, "y",
            1, "z",
            0, "a",
            1, "x");
    IntSeries sortIndex = new IndexSorter(df).sortIndex(Sorters.sorter(1, true));
    new IntSeriesAsserts(sortIndex).expectData(3, 0, 4, 1, 2);
}