I'm struggling a bit to make sense of it but I'll have a crack at an explanation.

The horizontal axis is the tote price of the favourite?
The vertical axis is the chance each favourite has of winning based on the moving scale of the price of the favourite (horizontal axis)?

So essentially in a 5 horse race the chances of all 5 horses winning are virtually equal when the favourite has odds of $3.70, correct?

It's strange that it has so many spikes, I thought with 3 years of data it would all be relatively smooth charts. Although there probably haven't been many 5 horse races in that time either.