Seeing as cellular service has a lot to do with the propagation distance of waves, I assume you could add a set of data to that and show that the less dense the population, the higher the nationwide average cost of cellular coverage. So as people per square mile goes down, average cell phone plan cost goes up. This is just a hypothesis, somebody with more time feel free to do the extrapolation

That link did not have average cost for either canad or australia, both of which are less population dense than US and which I expect would have higher costs