This is pretty heady- but my composer friends should get a kick out of it. Below is a breakdown from the creator of it explaining the methodology.
"For this visualization, I’ve broken composition key data down by composer. The values are displayed as a heat map, which I’ve made into the shape of a piano keyboard. For the non-pianists, I’ve put a key (pun intended) at the top so that you can see what the notes represent. The top half of each note represents percentage of minor compositions in that key, and the bottom half of each note corresponds to major compositions in that key. The highest percentage for each composer is labeled. Each composer’s heat map is scaled the same (0 to 20%), so you can compare one composer to another. The N-values next to the names show the number of compositions I was able to include in my database for that composer - these are not comprehensive lists! However, the samples sizes are large enough that the probability distributions should be at least reasonably accurate (to within a couple percent). A few noteworthy (yeah, pun intended again) trends pop out. Beethoven, Haydn, and Mozart all show strong preferences for a specific key. Other composers, particularly Dvorak, Brahms, and Mendelssohn, show less favoritism; these three had no keys associated with >10% of their compositions, with Brahms having the smallest standard deviation (2.14%). All three composers whose most-used key was minor were from the Romantic era (perhaps they were sad about the Industrial Revolution). The least used key across the board was g# minor, with Brahms using it most (in only 1.5% of his compositions). Rachmaninoff was the only composer to produce more pieces in minor keys than major keys; Haydn and Mozart most frequently wrote in major keys. Finally (I’ll spare you the details of my Friedman test), Tchaikovsky is the closest to the notion of an average composer; in other words, his probability distribution most closely resembles the distribution created by averaging all ten composers together." Lovingly shared from here