It's just a parametric plot showing two values changing over time. I suppose it's not too common for noisy data like this, but parametric plots are pretty commo

It's just a parametric plot showing two values changing over time. I suppose it's not too common for noisy data like this, but parametric plots are pretty common. The reasoning behind it is probably to emphasize that in most cases course length and average speed both increased over time, rather than being correlated.