Having played with horizontal polargraphing, I think you'd be better off building a standard Cartesian rig instead. A cheap Chinese Uno/CNC Shield/Stepper Drivers kit can be had for around $20. Load GRBL on the Uno. There are many free tools to generate g-code for GRBL.

A horizontal polargraph can be done, but the "bottom" corners of the drawing area (the corners far from the motors) can't be reached. Perhaps you'd come up with a solution I missed, but it would take time and ingenuity. I'd throw $20 at the problem instead.

The ZarPlotter guys solved the horizontal problem by adding two more motors, BTW, but they haven't released their code. Even if it was available, I'd still rather make a simple CoreXY Cartesian machine (it's only two motors, and you can use GRBL and any g-code tools you want).
http://www.zarplotter.com/

If you're rich, there's Tooli - http://toolbotics.com/ and their software http://www.art2gcode.com/

Assuming the question is in regards to the plotter realm (or the zen garden), I don't know of any affordable/ready-to-go CoreXY kits. There's plenty of 3D printer designs to get inspiration (and parts) from, though. The original CoreXY site has two sample implementations, one rigid and one less so. Somewhere between the two implementations is a good/simple plotter.
http://corexy.com/implementation.html

This plotter I made is not CoreXY --

-- but the way it's arranged would be good for the zen garden. Everything is on a the same plane, so fitting it under glass would be easy. I don't know how much influence the magnet/glass/ball/sand would have on the carriage, but I'd guess the zen garden would need more rails where I got away with string tension. The project I want to do for Maker Faire is similar to the zen garden in terms of magnets moving things above, and I plan to do a CoreXY rig similar to the plotter above.

For software, all the mainstream firmwares (GRBL, Marlin, Repetier, Smoothieware) support CoreXY now. Any software that generates standard g-code is useful. Most of the time, I've been using Rick's PenPlotter sketch for g-code generation.
https://github.com/RickMcConney/PenPlotter

I use Universal GcodeSender to send to GRBL:
https://github.com/winder/Universal-G-Code-Sender
Or, for over wifi, I use GRBLWeb, though sometime soon maybe LaserWeb
http://xyzbots.com/grblweb.html
https://github.com/openhardwarecoza/LaserWeb3

A simple CoreXY plotter implementation:
http://suburbnerd.blogspot.com/2015/01/dnd-map-plotter-part-2-plotter-hardware.html
The zen garden is a case where using those cheap 28BYJ-48 motors would be a good idea, 'cuz they're silent, slow, and the gear train slop doesn't matter.

For the garden, the arrangement used in EMSL's WaterColorBot might be worth considering:
http://shop.evilmadscientist.com/productsmenu/605#