@Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int w = getWidth();
        int h = getHeight();

        // Draw ordinate.
        g2.draw(new Line2D.Double(PAD, PAD, PAD, h-PAD));

        // Draw abcissa.
        g2.draw(new Line2D.Double(PAD, h-PAD, w-PAD, h-PAD));

        // Mark data points.
        g2.setPaint(Color.red);
        for(int i = 0; i < positions.size(); i++)
        {
            g2.fill(new Ellipse2D.Double(w/2 + positions.get(i).getX(), h/2 + positions.get(i).getY(), 4, 4));
        }
    }