Instructables
video Spiro-AR-graph-DUINO
You have seen the Drawbot, this is less capable but also easier to create!

This project is a sequel to my first instructable:  http://www.instructables.com/id/Arduino-Robot-for-lowest-cost/

No need to learn code and mess with processing files.  So easy your grade schooler can create their own designs!

Materials:
1) Foam board
2) fishing line or other string
3) Arduino with Scratch 4 Arduino interface
4) 2 continuous rotation servo motors - I used GSW S35
5) miscellaneous K'nex parts.  I love prototyping this way.  The video shows the fifth iteration of the pen holder.

Things I learned along the way:
A slant of approximately 10 degrees seems ideal
By using a smaller shaft I got better resolution/repeatability.  Since I was using continuous rotation servos I used time to control distance moved by each axis.  I may talk to the guru's at Citilab and see what would work better.
Connecting to the servo horn can be tricky.  I super glued wood dowel to a horn that I screwed to the horn mounted on the servo shaft.