public void testGetDomainAxisIndex() {
        CategoryAxis domainAxis1 = new CategoryAxis("X1");
        CategoryAxis domainAxis2 = new CategoryAxis("X2");
        NumberAxis rangeAxis1 = new NumberAxis("Y1");
        CategoryPlot plot = new CategoryPlot(null, domainAxis1, rangeAxis1,
                null);
        assertEquals(0, plot.getDomainAxisIndex(domainAxis1));
        assertEquals(-1, plot.getDomainAxisIndex(domainAxis2));
        plot.setDomainAxis(1, domainAxis2);
        assertEquals(1, plot.getDomainAxisIndex(domainAxis2));
        assertEquals(-1, plot.getDomainAxisIndex(new CategoryAxis("X2")));
        boolean pass = false;
        try {
            plot.getDomainAxisIndex(null);
        }
        catch (IllegalArgumentException e) {
            pass = true;
        }
        assertTrue(pass);