For a non-linear plot in a single equation, use
y=abs ( cos ( ((x-5)/20)*4*pi))
Then you can convert the cos wave to a triangular form by adding harmonics of
the cos, like they do in a Fourier synthesis.
(add 1/3*(cos(3*N)) + 1/7*(cos(7*N)) + 9, 11, 13, 15.....)
https://en.wikipedia.org/wiki/Triangle_wave
