For completeness in the discussion on the defensive spectrum here are the other positions graphed with all of the outfielders grouped together (this was necessary since the Lahman database I am using does not differentiate outfielders until 1996).
As you can see early in the century - until the 1930s - outfielders held a slight advantage offensively and were then generally eclipsed by first baseman. Once again this may be attributed to the need for good fielding first baseman in an era where bunting was more frequent. It's interesting that catchers were at the bottom of the offensive ladder until the teens when they eclipsed shortstops, who have for the most part remained there ever since.
Since I have data for 1996 by outfield position I thought I'd share that as well.
As you would expect DH's rule the roost while centerfielders come in last. Right and left field have jockied for second. The averages over this time span are:
Based just on this data using the (flawed) assumption that defensive value is inversely proportional to offensive output I would create the following defensive spectrums by era.
[ OF - 2B - 1B - 3B - SS - C ]
[ OF - 1B - 2B - 3B - C - SS ]
[ 1B - OF - 3B - C - 2B - SS ]
[ DH - 1B - OF - 3B - C - 2B - SS]
[ DH - 1B - LF - RF - 3B - CF - C - 2B - SS]
Of course, centerfielders were always more valuable defensively and sought after than the other two outfield positions and so it's not really fair to lump all three together. As a result centerfield should be placed farther to the right by one or two places in each of the periods before 1996. It's interesting to note the shift of second base to the right and first base to the left over time.