Make an Arduino-Controlled Drawbot by Shawn Wallace E-Book
This book describes how to build and program an Arduino-controlled machine for drawing algorithmic compositions on walls with a marker. The mechanism consists of two stepper motors with spools attached; the motors are attached to either side of the drawing area and a marker is suspended between them attached to fishing line.
The motors pull the marker along the wall; with a little trigonometry you can draw straight lines, circles, or Bezier curves. The book builds up a library of drawing elements that can be combined to draw almost anything. The book accompanies a kit available in the Maker Shed, or all the parts can be sourced separately from common parts.