### Plotting a 2d function

This should be easy, I want to make a nice colour plot of a 2d function where colour denotes the function value.

I can do

y=var('y')

f=y^2

plot(f)

and I get a nice plot.

If I try

y=var('y')

f=y^2+x^2

plot(f)

Then sage tells me it can't find the variable y. Which is ridiculous, since y is in the same place as when I tried to plot a 1d function of y. Anyway, I tried googling to find a function within sage to achieve this (without resorting to excessive matplotlib syntax) and had no success so thought I'd ask a question here. ~~Intuitively the first ~~Intuitively, the second case is hardly more complicated than the ~~second case ~~former so there should be a simple function that can realize this for me.

Many thanks!