It does appear that the first two years the trendline seems relatively flat overall (slightly down with extremes like Nebraska, but generally with a trendline hovering just above the 100% number). I guess what I'm interested in is how other teams fair in this category, ie, is the trendline for most offenses somewhat downward as defenses start figuring out tendencies, the majority of plays, get into conference play, etc. Obviously, a team like MSU this year doesn't see that, but it feels like for the most part, especially with Michigan for the past 6 years, the offense has always been generally better looking to start seasons and then production has slowly tapered off as defenses started figuring things out.
It would just be interesting to say, take the last 3 years (or 5 or 6 or whatever) and look at trendlines in single seasons of other teams, and see how that linear slope compares. My guess is that Michigan has a more negative slope than most other teams (I have my reasoning for why I beleive that is), but my feeling is that you'll likely see, as a whole, B1G teams with negative slope in their linear trendline (my guess is somewhere between the 2011 slope and 2013 slope, likely closer to the 2011 slope).
EDIT: an easier method would be just to look at 2013 season and look at the B1G as a whole. You can see the trendline of each team and then average them together to get a trendline for the entire B1G, possibly with the all the teams stacked behind that averaged trendline. I actually think that would be a more effective approach for you, as it would show how far this year is from the norm.