A few questions/comments about the Polargraph ...
I notice that the drawings seem to be on circular arcs with one of the motors as the arc centre which means that the lines (but not the complete image) are "upset" relative to the paper.
I presume you could counter that by pinning the paper on at a different angle ?
I presume it could be programmed to move the pen along straight lines?
Have you considered using thicker pens for greater covering capacity?
...R