Picture:
f(x) = -(xxx +x + 1)
(0,0) = center
color(x,y) = arctan2(x, f(x))
lum(x,y) = sin(hypot(x, f(x))/alpha)

[2020]
See also Who or what is Monica and why so much notice from SE users?.

[Oct 2024]
Its been 5 years now since the issue began. About 3 months later, Monica left.

Even though I used chux - Reinstate Monica for much of that time, I am reverting to chux.
I am sorry to see a better resolution did not unfold.