Thank you so much. One more thing: How can save teh
> graph in a file and then embed it in a Tkinter canvas
> widget,...??
you don't need to first save the graph -- though you can. You can
embed matplotlib directly. See
http://matplotlib.sourceforge.net/examples/embedding_in_tk.py
http://matplotlib.sourceforge.net/examples/embedding_in_tk2.py
At this point, it would be a good idea for you to spend some time
reading the documentation on the website, as many of these ideas are
covered there.
JDH