public void testGetXLowValue() {
        XIntervalSeries s1 = new XIntervalSeries("S1");
        s1.add(1.0, 2.0, 3.0, 4.0);
        assertEquals(2.0, s1.getXLowValue(0), EPSILON);
        s1.add(2.0, 1.0, 4.0, 2.5);
        assertEquals(1.0, s1.getXLowValue(1), EPSILON);