Note: It seems LEGO has a number of different tread standards. This one is for the "large" ones.

The kids and I built a XY cartesian system with motors which could draw some nice patterns (but operated with the remote control so not so fancy plotter but good enough do demonstrate the principles of design).

Unfortunatly I didn't take any pictures of the plotter before it was trashed for other inventions.....

The pen body was screwed into this adapter and the inner working of the pen is movable so that it's weight gives the pressure towards the paper but can't fall out of the adapter. I guess it's a matter of finding the right pen to get it to work for you though.... but again, it's the principle that is important... and this idea is to have the pen mounted to the treads.