Pretty full-featured, and a reasonable API. The only shortcoming I can see right now is that it doesn't anti-alias line charts. Although at the daily resolution I use it's not such a big deal.

Google Docs version: http://willpowerisfo...m/daily-weigh-in/

Image_Graph version: http://willpowerisfo...acking/?user_id=1

By the way, for anyone into WordPress: All that is set up as a plugin. Adds user preferences to capture goals; creates the page to display all users -- if they set themselves as "public"; and individual users' chart, goals, and current-day data entry.

I don't plan on actually selling the plugin at this point, but structuring it that way future-proofs it against WordPress upgrades.