[mhermher]

I think it's just that they are plotting sum vs ratio of two random variables. Try this (in R):

a <- runif(1000); b <- runif(1000); plot(a + b, log(a/b))

Also, they likely have a low-end cutoff (notice their x axis starts at 10^4. If you do the same to the above plot, you get even closer to that exact shape. Try:

plot(a + b, log(a/b), xlim=quantile(a + b, probs=c(0.2, 1)))

[_pastel]

Note also their x axis is on a log scale, which makes the edges linear instead of curved. E.g.:

plot(a + b, log(a/b), xlim=c(1, 2), log="x")

[mhermher]

ah, thanks. I missed that.