The 3D graph shows the motion of a free-swinging pendulum.
The graph on the upper right is the x-coordinate
(red axis on 3D graph) vs. time, and
the lower right graph is the y-coordinate (green axis on 3D graph) vs. time. The two
time graphs together show how the greatest excursion of the pendulum moved between
the x- and y-coordinates. The pendulum swung in an ellipse that rotated in time.