Norbert Heinz cannibalised two optical drives for their parts, then hooked them up to a Raspberry Pi via a couple of H-bridge chips before adding an arm with a pen. This gave him a drawing mechanism that could be programmatically moved on the X and Y axis. He then added a stepper motor to enable the pen to be lifted on the Z axis and voila, he has managed to create a plotter. You can read more about his build, and perhaps replicate it, on his blog.