It is actually the state of the pendulum which is being plotted. The state plot in this case came out to be elliptical and repeated itself with each cycle of the pendulum motion. We will refer to the completed path as an "orbit" in phase space. The time required to complete an orbit is called the orbit "period". The fact that this orbit is closed, meaning that it finishes at the same state from which it started is characteristic of systems which have no energy transferred in or out, as is the case with this imaginary pendulum. The fact that the orbit is elliptical rather than circular or elliptical with the major and minor axes reversed is simply a matter of the scales chosen for the position and velocity dimensions. Different choices of scale would produce a different orbit shapes but they all would closed and of unchanging height and width.