You can see that Clemens almost followed an opposite pattern from all other pitchers in the league. As Andrew notes though, there seems to be a lot riding on the quadratic fit and average values when we know that Clemens has been anything but ordinary throughout his long career.
Graphing Without Smoothing
For fun, I tried graphing the ERA data for Clemens against the ERAs for the 16 most recent hall of fame pitchers (that I could get data for). My thinking was the hall-of-famer performances might be a better indicator of what should be “normal” for great pitchers. The results are a little less compelling. However, one thing to note is that most players who played past age 40 saw an increase in ERA while Clemens had a pretty significant improvement in ERA from age 40 to 43.
Whether this is due to performance enhancing drugs or just a change in pitching strategy, coaching, or some other factor, I can’t say. There’s probably only a few people who can know for sure.
Anyways, if anyone has a different take on the data, I’d love to hear it in the comments.