Whoops, I thought that the P had to represent something that started with a P!

I suppose if P can truly represent anything, then the graph demonstrates a relationship of any thing to P, with the thing becoming more numerous and clustered as P increases.

Therefore, we could impose any positive linear relationship involving some kind of small groups or clustering on the part of the dependent variable. Another example that comes to mind is increased urbanization with either an increasing population or simply time progression (more P words but still).