Second, I actually find the most interesting fact to be completely unmentioned by the author: that in the first chart (greatest absolute changes), of the 30 biggest swaps, only two were female->male, while the other 28 (over 90%!) were male->female.
It's a remarkable fact -- makes me think of how it's historically been much more acceptable for women to adopt men's styles of clothing, but much less accepted for men to adopt's women's styles of clothing. And with names being equally "fashionable", this fits the same trend.
This also explains what I'd observed but never articulated about late 19th- and early 20th-century fiction: how come so many men have what are to me "women's names"?
Utterly fascinating.