Just want to clarify a little. Since x^2+y^2 = 1, the projection of the torus onto the xy-plane is a circle. The same is true for the zw-plane. A position on t

Just want to clarify a little. Since x^2+y^2 = 1, the projection of the torus onto the xy-plane is a circle. The same is true for the zw-plane. A position on the torus can be specified by specifying the position in the xy-plane, and the position in the zw-plane. Since the projections are both circles, those two positions can be represented by one angle each. And this is exactly what you see on your screen. The right-left axis is the xy-angle, and the top-bottom axis is the zw-angle. The edges of your screen correspond to an angle of 180/-180 degrees.