Add values to a specified series in a DynamicTimeSeriesCollection

The program will receive data every second and draw them on time Series chart. However, once I create two series, I cannot add new value to it. It displays a straight line only.

How do I append data to a specified series? I.e. YYY. Based on this example, here's what I'm doing:

...
    // Data set.
    final DynamicTimeSeriesCollection dataset =
        new DynamicTimeSeriesCollection( 2, COUNT, new Second() );
    dataset.setTimeBase( new Second( 0, 0, 0, 1, 1, 2011 ) );

    dataset.addSeries( gaussianData(), 0, "XXX" );
    dataset.addSeries( gaussianData(), 1, "YYY" );

    // Chart.
    JFreeChart    chart = createChart( dataset );
    this.add( new ChartPanel( chart ), BorderLayout.CENTER );

    // Timer.
    timer = new Timer( 1000, new ActionListener() {
        @Override
        public void actionPerformed ( ActionEvent e ) {
            dataset.advanceTime();
            dataset.appendData( new float[] { randomValue() } );
        }
    } );
...

private JFreeChart createChart ( final XYDataset dataset ) {
    final JFreeChart result = ChartFactory.createTimeSeriesChart(
        TITLE, "", "", dataset, true, true, false );
    final XYPlot     plot   = result.getXYPlot();
    ValueAxis        domain = plot.getDomainAxis();
    domain.setAutoRange( true );

    ValueAxis range = plot.getRangeAxis();
    range.setRange( -MINMAX, MINMAX );
    return result;
}

Assuming you started from here, you've specified a dataset with two series, but you're only appending one value with each tick of the Timer. You need two values for each tick. Here's how I modified the original to get the picture below:

final DynamicTimeSeriesCollection dataset =
    new DynamicTimeSeriesCollection(2, COUNT, new Second());
...
dataset.addSeries(gaussianData(), 0, "Human");
dataset.addSeries(gaussianData(), 1, "Alien");
...
timer = new Timer(FAST, new ActionListener() {

    // two values appended with each tick
    float[] newData = new float[2];

    @Override
    public void actionPerformed(ActionEvent e) {
        newData[0] = randomValue();
        newData[1] = randomValue();
        dataset.advanceTime();
        dataset.appendData(newData);
    }
});