The poster's designer, Larry Gormley, says he worked on the piece "on and off for the past five years:"
"I started the data collection phase by viewing copies of Variety magazine from the 1930s in the libraries at Harvard University. Over the next four years, using various sources, I collected a database of approximately 3,000 films. Each film was classified according to genre, release date, and, most importantly, the relevancy for inclusion in the final graphic. Additional information about the film was collected such as director, box office sales, and actors. Some souces used to collect this data include: the AFI database; various periodicals; and books and web sites by recognized film critics and experts in a particular genre."
Next he evaluated the candidates using four different criteria: critical acclaim; importance within a particular genre or director's body of work; awards; and box office success. He decided that 2,000 was the maximum a readable piece could comfortably hold, so he winnowed his shortlist down by a third, until he arrived at this final graphic.
It's tough to see the details in that full version (I encourage you to click over to the poster's home at HistoryShots.com, where you can get a much more thorough look and buy a copy) but I think you still get a sense of something that's great about it: the way it charts cyclical trends throughout the years just by the way genre groupings swell or shrink over time. You can see the Western's biggest heyday in the 1950s and '60s, and compare it to the rise of fantasy films like "The Lord of the Rings" and "Harry Potter" franchises in the last decade.
It looks like the musical never again approached the heights of its popularity in the 1940s. On the flip side, the animation genre has absolutely exploded in recent years, with the vast majority of the genre's included titles coming in just the last two decades.
You could quibble endlessly about the movies that did ("Rapt") or did not ("Brokeback Mountain") make the list -- but I think that's part of the fun of the piece. It would make a great conversation starter for the cinephile or blogger in your life whose birthday happens to be rapidly approaching *coughlikeminecough*.
Purchase "The History of Film."