You've already seen that you can draw a line in your PyGTK app like this:
widget.window.draw_line(xgc, x1, y1, x2, y2)
The color, line thickness and line style will all be taken from whatever you've set in xgc, your graphics context.
You can use that to make some whizzy color-changing graphics -- lines constantly redrawing in different positions and colors. You could make a screensaver! See the rest of this excellent howto by ace coder Akkana Peck at LinuxPlanet.