I first want to give credit to mttorley and the Circular Rainbow brush, as this one is essentially a replication in concept. Wanted to try to mess with the asymptotic behavior to see what options were available. Tried to steer away from trig by using just the canvas pos, and the rotation brush parameter became an issue. The controller for the rotation ended up a wall of absurdity. Other parameters like size, color, opacity, and scale use a simpler controller (move this one around for other different effects). Luckily the erratic behavior at the axes is almost nonexistent, although sometimes gaps will appear (shown in example) when the brush stroke is really fast. Would like to see someone try it for a perspective piece to see how it functions, or with some of the pen parameters I can't test. Hope you like it.
Again, thanks to mttorley. If you happen to spot this, can you give it a whirl for a critique?