How to draw an arrow using matplotlib

Dear John, I see your points. However, after I changed to

    > using add_artist. This is still nothing on the canvas. I
    > think that there is some problem with the draw function of
    > my arrow class. I have no idea what else should be
    > there. At the moment, only one call in my case as below.

    > def draw(self, renderer): self._stem.draw(renderer)

    > Could u tell what else should be called?

Your drawing function looks fine. Could this be a problem with axes
limits. Make sure the x and y limits are set properly with the xlim,
ylim or axis commands. The limits are normally autoscaled by calling
plot, but you are not using it so you have to take care that the
limits are set. After you get a working prototype, I can help you
with the autoscaling part.

If this doesn't fix your problem, email me your, and
test code as attachments off-list and I'll take a look.