This is just perfect! Using a Magna Doodle as an output device for a computer is just brilliant.

What do you get when you mix a 1970’s style analog chart recorder, an 8-bit microcontroller, and a Fisher-Price Doodle Pro? A truly 21st century toy: An analog PlotBot with e-paper display technology!
Our machine is based around a vintage analog X-Y data recorder. Its original purpose in life was to perform basic laboratory data collection, plotting two voltages against each other, and was one of the primary tools for that purpose right up until computers took over that job in the 1980’s. Because they were once so common and are now generally obsolete, it’s quite easy to get one of your own. There are usually several under $50 on eBay at any given time, and that’s where we got ours. …
The other major modification that we’ve made is that we’ve replaced the pen and paper with what seems like out of reach technology: an inexpensive and readily available e-paper display: the panel from a Doodle Pro. …
The Doodle Pro is a descendent of the Magna Doodle, a classic children’s toy dating to 1974. (I’m not sure what makes this a "Pro" anything, however.) It uses a simple magnetophoretic display, where ferromagnetic particles are suspended with near-neutral buoyancy in an opaque, viscous white liquid. Using a magnetic stylus, you can attract the black particles to the top surface, or with a magnetic "eraser" on the bottom side, pull the particles away, leaving only the white liquid visible.

Evil Mad Scientist Laboratories - An AVR-based Analog Plotbot with an E-Paper Display