plot parabola using imagesc

Hi there, I want to plot a parabola (equation, range, domain does not matter) using imagesc. I'm unsure how to even aproach this, except that id have to create a lin space for my values of x and y.

After this, I want to create a plane that intersects with my parabola that has some width to it so i can see how it seperates space.

Im so lost and would appreciate anyones help.

For example,

say i have the equation y = x^2

and want to create a plane y = 4 that goes through my parabola and has some width to it.