Well to be quite honest with you I never did make such a thing as the plotter you requested , but I'm generally interested in mechanical designs and motions.
Now back to your work,
The idea depends on having 4 rods, 2 are screws only rotating, and the other two will not rotate but will only move left or right , upwards or bottom.
So let's imagine this, you will have your board with the paper above it, right above it place your first screw horizontally along the board at the bottom of your board. anf tie it using a bolt to a rod placed horizontally right above it.
this way when the screw rotates the vertical rod will move right or left
Above this place the vertical screw at the right most of your board and on top of it tie the horizontal rod using a bolt to it. this way when your screw rotates the horizontal rod will move up or down.
Whats left is to tie your pen between the vertical rod and the horizontal rod
I'l try to skecth something and pm to u as soon as possible
Hope i did really help.
Regards