I made a plot clock too... Thanks joo! http://www.thingiverse.com/thing:248009

I had a hard time with a few things, so I documented my process in the instructions. Hopefully these will help someone else.

My modification is really minor, and it looks like there are others that do the same. I created a scad file of the part that connects to the arm servos that fits the single horn that came with my servos and used the following projects in addition to my horn part. I also modified the ino file to use some different constants that made it easier for me during the calibration process. See the instructions for more on this.

1. 3d printable parts: http://www.thingiverse.com/thing:250204
2. Expo marker arm: http://www.thingiverse.com/thing:261999
3. Battery holder: http://www.thingiverse.com/thing:456900

Video of it in action:
https://goo.gl/photos/qACTKfDJhuQvSEaRA