I wonder how this compares to a two-wheel plotter; I saw one of those demoed and it made surprisingly crisp drawings for how simple it was.

If you're not familiar, take an inclined drawing table, have a wheel at each top corner with cable (or string) running to the pen, making a triangle. Using a servo to retract or release the cable will change the length of that side, moving the point of the triangle. The one I saw had a simple spring-loaded solenoid for lifting the pen.

Biggest downside is speed is limited by how fast gravity can keep a releasing cable taut.