Silly me. I was comparing starters to all pitchers. We all know that there are three kinds of pitchers: starters, relievers and Joba Chamberlain.
I isolated the starters with a simple 100 IP cutoff. I can similarly isolate the relievers by requiring no more than 6 starting innings pitched. This captures 86% of the relief innings, with less than 1% contamination by starter innings.
Graphs for relievers are below:
There is not a whole lot of difference between the known relievers and the "all pitchers" figures. However I can't help note just how similar all lefty relievers' repertoires really are. If this is evolution, I hope evolution doesn't happen to the general pitcher population.
Then again, the data I'm using does not take account of two-seam fastballs or of sinking fastballs. But still, all of the crafty lefties I can think of are starting pitchers. As I mentioned in an earlier post, there has been a steady increase of lefty starters in the past decade. So maybe clubs are doing a good job of getting their best lefties into the rotation. The lefties who throw smoke but can't learn a pitch other than a slider end up in the bullpen.