There are four graphs below. Each one shows the slugging percentage (SLG) divided by the league average for all 8 every day fielding positions. Each data point is a five year average. The first two graphs are the AL and the next two are the NL.
Some interesting trends:
-Shortstops have been rising quite a bit in both leagues since the 1970s.
-2B men started declining in the AL in the 1940s, then starting rising again in the 1960s. But even now they have not reached their earlier peak. They started declining in the 1920s in the NL but then started back up in the late 1950s.
-3B men started to rise in the AL in the 1920s and in the 1930s in the NL. But they have tailed off in the AL since 1980.
-1B men had a big spike in the AL in the 1920s and 1930s. In both leagues, in general, they have been high but have fluctuated.
-CFers seem to have been in decline since the 1970s in both leagues.
-LFers seem to have been in decline in the AL for some time but it does not seem that way in the NL.