That chart actually force me to look at the key multiple times. Perhaps I am tired but I kept wanting to use the top key "female" for the top line within the chart....strange.
Who knows why the author used green and yellow - feels 'blue for a boy and pink for a girl' is outdated and sexist, or just likes green and yellow better, or they're the default from their charting program, or a chosen colour scheme for the blog. But there's no doubt (for readers in the English-speaking world and who knows how many countries) what would be clearer, to the point of hardly needing a key.