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.sortByValues(SortOrder.DESCENDING);
// check key order
assertEquals(data.getKey(0), "D");
assertEquals(data.getKey(1), "A");
assertEquals(data.getKey(2), "C");