The obvious way to make the charts easier to interpret would be to use pink and blue.
Who knows why the author used green and yellow - feels 'blue for a boy and pink for a girl' is outdated and sexist, or just likes green and yellow better, or they're the default from their charting program, or a chosen colour scheme for the blog.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.