https://github.com/openwebwork/webwork-open-problem-library/blob/master/Contrib/PCC/BasicAlgebra/FunctionBasics/FunctionOrNotByGraph20.pg

It was not plotted as two sqrt functions, but rather as a shifted polar form converted to Cartesian. You can adopt this to plot any x(t),y(t). For circles, this would be much simpler than parabolas.