The detail in the plot is unusual (and cool). Thanks.

I'm sure you know this (or aren't surprised by it) but resistant measures of symmetry and 'tailedness' could very well show similar relationships. I've plotted Galton's skew (using 10/90 instead of 25/75 but no matter) for X and the tail-weighted index (as the kurtosis fill in) for Y and I see a similar relationship for Weibull and lognormal. I realize if you have large sample sizes and believe there are no blatant outliers then you'd like to use skew and kurtosis, but for my purposes I'm dealing with plenty of outliers (regardless of sample size).