I'd like to create a graphic image on the fly based on user input, and then present that image as a PNG file (or jpg or gif if necessary, but PNG is preferred).

This is actually for an astrology application; what I'd like to do is generate the chart in PNG for display.

Python or Ruby is fine; in fact, the library available may determine the language I use.

Update

Here's an example image:
Example horoscope graphic chart

python ruby graphics png
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