I like how the bar chart has a shadow 'total' behind the two groups
Where you see both volume, distribution, percent, as well as the trendline (white) where you can see how the distribution changes - how the total count changes, and easily understand any data point at any given spot.
Who knows why the author used green and yellow - feels 'blue for a boy and pink for a girl' is outdated and sexist, or just likes green and yellow better, or they're the default from their charting program, or a chosen colour scheme for the blog.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.