data.addValue("C", new Double(1.0));
        data.addValue("B", null);
        data.addValue("D", new Double(3.0));
        data.addValue("A", new Double(2.0));

        data.sortByKeys(SortOrder.ASCENDING);

        // check key order
        assertEquals(data.getKey(0), "A");
        assertEquals(data.getKey(1), "B");