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Then replot it if you want. HR's per 500 AB have bounce around from between 13 to just below 15 since 1994, with 2006 not being too far off peak. While significant, it is not nearly as big a jump as you see pre 1994.
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Some things you aren't considering:
1. Expansion has a great effect on the talent pool, especially pitching.
2. Exapansion in 1993, and then in 1998 caused a noticeable effect in the rise in HR production.
Take a look at this chart I whipped up:
Prior to expansion 1979-1992 average: 9.36
After 1993 expansion, from 1993-1997: 12.20
After 1998 exapnsion, from 1998-2007: 13.81
(obv, 1987 was quite an aberration, cover it with your thumb on the chart, and you see three distinct groupings.)