Rotated fluids generating a paraboloid

Topic:
Parabola
The picture shows two different fluids rotating. Theory predicts that the interface should be a parabola when viewed from the front or side (not the top). A parabola is constructed using a focus and directrix. Tasks 1. Adjust the focus and directrix (controlled by two points) to fit the interface. 2. Adjust the rotation slider until the standard form of the equation for a parabola results (only one quadratic term).