This was constructed to illustrate the Tautochrone curve: a basic physics concept wherein an object will take the same amount of time to reach the bottom of the curve no matter where you drop it (assuming uniform gravity.) Cool, right? Now that I’ve explained it, you get it, right? It’s even easier to understand if you look at the little graph in the upper right hand corner which, obviously, tells you the relationship between (s)peed and (t)ime.
The Y axis actually measures arc length. Visualization is about communication, and since this targets those unfamiliar with physics it does a poor job by using potentially misleading labeling simply because it is agreed upon by the professional community.
Jargon seepage is dangerous.