Doubt in graph of wave function

Question

The 1s orbital in polar coordinates is given by: $Ψ=2(1/a_0)^2*e^{-r/a_0}$ I am confused if $r^2$ here is $x^2+y^2$ or $x^2+y^2+z^2$(here z is nothing but $Ψ$ itself) .When my professor graphed this function,it looked like a sheet with a hump in the middle.If i substitute $r^2$=$x^2+y^2$ and plot the function, it matches the prfoessor's graph.Substituting the other option gives the wrong answer.But I learnt in the theory of spherical polar coordinates that $r^2$=$x^2+y^2+z^2$. Basically,I want to convert the polar system to a cartesian system.Where am I going wrong?