I'm a big fan of the google charts API - it draws much better-looking graphs than I would ever manage and all I have to do is assemble the right URL to make it work. I recently got a feature request to add QR codes to joind.in, so that speakers and event admins could easily allow people to link in to a particular talk page.
To cut a long story short, joind.in already has short urls for particular talks, just in the form
http://joind.in/[talk_id] so I created the QR codes to point to these. The URL looks like this:
The parameters, in turn, are the chart size, the chart type, and the URL to link to - The image ends up looking like this:
Absolutely simple when you know how - thanks Google Charts :)