This thing is a remix of the already great design by Joo. It incorporates ideas from users corben33 and kghjfg.

A few enhancements:

1. Hole sizes are optimized for the hardware.
2. Clearances for the servos were increased slightly.
3. Thicknesses optimized for a 0.4mm layer height.
4. Hollowed the drawing plate to save material and time.
5. Changed the mounting method of the marker holder.

On a side note, I found a great place to get Arduino stuff. $7 for an UNO clone. Nice! http://www.banggood.com/Wholesale-Arduino-Compatible-R3-UNO-ATmega16U2-AVR-USB-Board-p-68537.html?p=S6191815187312015045

1. Print the parts.
2. Build the arduino program using https://github.com/9a/plotclock
3. Calibrate
4. Run

Helpful tips:
Use servo horns. I tried designing special arms, but they kept slipping.
For the wiper, I used a piece that is normally used on the leg of a chair to prevent floor scratches.