I'm surprised that according to the article, in jazz, some chords like D and A, which are mostly found in sharp keys, are more common than chords like Bb and Eb, which are usually found in flat keys.

I remember once creating a dataset based on 50 random tunes from the Real Book and sharp keys were less than 20% of the total (based on the key signature at the start of the score) so that graph in the article doesn't seem right.

Maybe the discrepancy is because modern jazz fusion tunes are under represented in the Real Book and those are usually more guitar-oriented, so perhaps more likely that the musician would pick a sharp key like D or A. As opposed to straight-ahead jazz were people try to accommodate for sax/trumpet/trombone etc.

Or maybe it's because chords like D or A can be dominants in minor keys that are flat keys, e.g. D in the key of Gmin or A in the key of Dmin. - EDIT I just realised that dominants are listed separately so this is not the case.

One more thing: according to the article, major triads make up more than 50% of chords in jazz... what? That's certainly wrong, most major chords in jazz are usually maj7th or 6th even when they don't have upper extensions. I think that what they actually meant is "major chords that are not dominants". But they used the label "major triad" instead.

The dataset is not “jazz,” it’s Ultimate Guitar’s tablature of jazz songs, so the data is very low quality.