This isn't what you're asking for, but:

For my plotters that use gcode and GRBL shields, (so, not my Polargraph based projects) I've been using GRBLWeb on Raspberry Pis with wifi dongles. I've got one plotter with a very old RP1 that's been handling the job fine.

http://xyzbots.com/grblweb.html

That probably doesn't help you if you want a Sandy Noble Polargraph™ system running. My programming skills are undoubtedly less than yours, and I have no idea how difficult it would be to make something like GRBLWeb communicate in Polargraph codes. Perhaps it's something you could take a look at, though.

So I got it running for the first time. Turns out that the Arduino needed it's own power. The Raspi wasn't able to drive it over USB.
The UNO mounts as /dev/ttyACM0

Connected successfully to /dev/ttyACM0 (<module 'serial' from '/usr/lib/python2.7/dist-packages/serial/__init__.pyc'>).
received: ?POLARGRAPH ON!
received: Hardware: 1
received: MC_UNO
received: Loaded width:732
received: Loaded height:884
received: Loaded mmPerRev:82.00
received: Loaded steps per rev:400
received: Loaded step multiplier:1
received: Loaded down pos:105
received: Loaded up pos:130
received: Recalc mmPerStep (0.20), stepsPerMM (4.88)
received: Recalc pageWidth in steps (3570)
received: Recalc pageHeight in steps (4312)
received: READY
Command 1/61132: C14,END (0%)
received: Basic Command executed.
received: READY
Ran 1 commands in 2.39 seconds: at 2.39 seconds per command, we'll finish in 16:29:58)
Command 2/61132: C17,3700,2810,2,END (0%)
received: maxLength: 5598
received: SYNC,3700,2810,END
received: Basic Command executed.
received: READY
...

How do I "home" it?
And how do I tell it what paper size I have and where it is located?
It is moving now, but not where I thought it should be.

So I recorded all the gcode that I use to set up my Polargraph and played it back. It works! 🙂 I just have to prepend that to the drawing code.
Still having issues with my power to the Raspi. Whenever the servo moves the connection gets lost. When I have it powered by my Macbook it is stable. That setup doesn't free up my Laptop then. 😉
So off to look out for a beefy USB power plug...

Just to test things out I have a power supply for the shield and the Arduino. Even then the Raspi stalls when the servo goes off.
I found a new problem though. The settings stored on the Arduino don't match up. When I dug into it I found out that the settings are off in the Controller now too. Whenever I upload the settings and retrieve them back I get different settings back.
Could my EPROM be broken? How can I find out where things go wrong? Compile the firmware with my settings hardcoded into it?

Using FIRMWARE_VERSION_NO = "1.2.1"

It gets even stranger. When I read out the values with the Arduino monitor the are correct, the controller messes things up. I wouldn't mind if the pen wouldn't end up where it shouldn't. I've measured everything twice now, after I have converted to fishing lines. I'll try to play with the values until it kinda works out.

Yes for different paper sizes.They all share the same machine settings though.

Thanks for all the effort you're putting into this project. I really appreciate it.

Coming back to the original idea of having a light weight controller for drawing svgs/gcode. I do all the vector work in Illustrator and save them as SVGs. If I were to use a dokument size of my machine and place and scale the lines there, there is no need to do the same in the controller. The cartesian coordinates are already there.
A light weight controller would only have to transcode the cartesian x-y coordinates to the "polar" a-b coordinates and send them to the machine. All it needs to know is the machine specs (saved in the eprom) and that the pen is in it's home position, right?

A worklow I had in mind would look like this:

• Create a workspace in Illustrator/inkscape the size of your machine
• Place and scale all elements in Illustrator/inkscape
• Export as SVG

• Import the SVG into the controller
• SVG is being parsed, beziers are converted to lines (arcs?)
• Path is optimized for travel distances (optional)
• XY cartesian coordinates are converted to AB polar coordinates
• Pen is placed on on the machine to the home position by the user
• Gcode is generated and exported to the machine or Raspi-draw-server

All that wouldn't really require a GUI as all the placements are done in Illustrator.
Only a machine setup would have to be done previously.
Being really optimistic, all of that might fit into an Illustrator/inkscape script so that you could draw right from either of those applications.;)
Inkscape already has a path reordering script.

Am I missing something in this concept? Seems that a workflow like that would be way simpler for anything that doesn't use the raster image functions of the polargraph.
All of the pieces are there, I just can't judge how hard it would be to reassemble them into something like that.

Hm... so I found this Primer on Beziers. But again I have no idea how I could turn any of it into code. Bezier.js looks promising as a starting point for an Illustrator script.
Do I wish I had paid more attention in math when I was in school... 😉

Do I get that right, you're moving away from Processing to Python?

So it'll be web based but not running on a LAMP stack. More likely in python or node.js? Sounds great to me. I will not ask when that will see the light of day... 😉

Meanwhile we have the send.py script to work with. I've added a notification with IFTT that will send a push notification on my phone when it's done drawing.
It's just a few lines of code for those wanting to have it:

import requests

    def close(self):
        print "Finished sending {:d} commands in {:0.2f}".format(self.total_lines, time.time() - self.time_started)

#       http://docs.python-requests.org/en/latest/user/quickstart/#passing-parameters-in-urls
#       curl -X POST -H "Content-Type: application/json" -d '{"value1":"File","value2":"Rows","value3":"Time"}' https://maker.ifttt.com/trigger/{your trigger name}/with/key/{your key}

        m, s = divmod(time.time() - self.time_started, 60)
        h, m = divmod(m, 60)
        d, h = divmod(h, 24)

        url = 'https://maker.ifttt.com/trigger/{your trigger name}/with/key/{your key}'
        payload = {'value1': sys.argv[1], 'value2': self.total_lines, 'value3': "{:02d}:{:02d}:{:02d}".format(int(h), int(m), int(s))}
        r = requests.post(url, json=payload)
        print(r.text)

        self.file.close()