I think there's a distinction that needs to be drawn here between data that only has two of something, and functions that only take two arguments. In the former case, you limit the things you can represent and generalize to, and everybody agrees that's not good. In the latter, though, there is the standard notion of folding that naturally extends your binary function to arbitrarily many arguments. In fact, I'd agree with what I think you're suggesting: it's better to define a binary function and let fold generalize it, than to have to roll your own multi-argument function every time, because then you and anyone else looking at it can easily know how it behaves. In other words, I think he got carried away by the end of the article, and his last "improvement" isn't an improvement at all.
(Of course, the objection to data having only two things doesn't hold either if it's a recursive data type -- cons lists, binary trees, etc... -- but you know what I mean.)
This is really informative, and very well done to boot. Great work!
The colour scheme struck me as a little odd, though. It goes from darker (more migration) to lighter (less migration) but then abruptly to dark grey (no migration), making it harder to interpret at first glance. It would be nice if it were somewhat monotonic: bright colour = more migration, darker/duller colour = less migration, dark grey = no migration.
(Of course, the objection to data having only two things doesn't hold either if it's a recursive data type -- cons lists, binary trees, etc... -- but you know what I mean.)