I've been calling it "error" (the difference between the left and right side of the equation). But if there is a better term to use, I'd like to know it.

I think "difference" is better, as "error" implies one side is correct and the other is an approximation. Specifically, you're plotting absolute difference z = |f(x, y)-g(x, y)| (or, worse, absolute error), not error.