Thank you! I made it like that purely for fun, this project was not intended to be useful, or have the best mechanical design :P

Using Gcode is a good idea, though it does seem a bit overkill for this simple application

Wouldn't you need the Arduino to do the inverse-kinematic calculations if you want to interpret gcode?

See fluidnc http://wiki.fluidnc.com/en/config/kinematics as an example where you can get kinematics going without too much effort. It also has links to more pen plotter implementations to learn from.