Nice , it's working quite well !
The only downside is that polar transform is a bit heavy on calculations, with only one in the graph that's ok. But let's say someone want to use several effects to make a pie-chart motion graphics, that should start to slowdown.
I was planning to look into this one day or another, using these expressions as a start : http://www.nukepedia.com/written-tutorials/expressions-101 and translate it to shadertoy code.
Anyway it's already cool, and I'm sure it will be helpful as it is for many people !