You can create a graph with the angle as the x-axis and the speed as the y-axis. But since traveling 360 degrees brings you back to the start, you can sew together the vertical lines where x is zero degrees and where x is 360 degrees. This makes a cylinder. The cylinder doesn’t directly reflect physical reality — it isn’t showing paths that the pendulum traces — rather, each point on it represents a particular state of the pendulum. The cylinder, along with the laws that determine the paths the ...