I usually use the following trick to show a 3D plot (where I have $f(a,b)$ and I want to show the values of $f$ versus different values of $a$ and $b$) in a 2D plot:
I'm wondering what are some ideas for showing a 4D graph in a 2D plot where I have $f(a,b,c)$ and I want to show the values of $f$ versus different values of $a$, $b$ and $c$.
You can simply assume: $a$ and $b$ are continuous and $c$ is discrete
I was not sure which forum is appropriate to ask this general question about visualization but I'm hoping to find some advice from math perspective here.