Changes in the Game

I picked up a copy of the late Leonard Koppett's The Thinking Fan's Guide to Baseball this weekend. In reading his chapter on changes to the game (expansion, a livlier ball etc.) I wanted to take a quick look at the changes along the axes that he mentions; runs scored per game, batting average, and homeruns per games. Using the Lahman database I created the following quick graph. The trendlines for Runs/G and HR/G use a moving 10 year average.
I'll have more comments on the book and Koppett's view of these changes along with his take on sabermetrics later this week.