DialPlot p = new DialPlot();
        DialBackground b1 = new DialBackground(Color.red);
        p.setBackground(b1);
        p.addChangeListener(this);
        this.lastEvent = null;
        b1.setPaint(Color.blue);
        assertNotNull(this.lastEvent);
        
        DialBackground b2 = new DialBackground(Color.green);
        p.setBackground(b2);
        this.lastEvent = null;
        b1.setPaint(Color.red);
        assertNull(this.lastEvent);
        b2.setPaint(Color.red);
        assertNotNull(this.lastEvent);