Seems a necessary milestone for anyone starting with robotics. Whatever...
Open source hardware (arduino), Adafruit's motor shield, and a lot of help from the many places dealing with stepper motors and servos, among them Tom Igoe's.
Disclaimer: more than one discarded printer and/or DVD rewriter was maimed and eviscerated during construction.
The pen up/down motion is controlled by a small (4g) servo. The latitude stepper is 1.8 degree per step (200 steps/turn), the longitude one is a geared down 7.5 degrees per step (48 steps/turn), both driven with half-stepping.
Lessons learned: use shielded and grounded twisted pair for cabling!
PWM for the servo and microstepping does produce interferences.
The spring drawing the pen to the egg is a bit too strong, so we lose some precision
More on this later. I started adapting someething inspired by one of the 5-D gcode interpreter from reprap, then I switched to building an x-y plotter...