I've made a video showing how it works. My camera unfortunately cut it off after a few mins, but the idea should be clear. There are other movies from earlier revisions of the hardware/software too. And yes, the plotter is slow but precise.
For the people who want to copy my setup: Due to the random parts ripped from hardware I had around, I can't exactly give you a step-by-step howto. If you know php and can program an AVR, you can probably make sense of the source-code, which can be downloaded here.