What does this graph mean?

Mark Liberman:

It’s clear that they were looking for more evidence in favor of Prof. Twenge’s “me generation” theme. I’ll leave it as an exercise for the reader to sort out the actual relationship between their data and this conclusion; but in my opinion, the prima facie case is very weak for the conclusion that (as the USA Today story put it) “An analysis of words and phrases in more than 750,000 American books published in the past 50 years finds an emphasis on ‘I’ before ‘we’ — showing growing attention to the individual over the group”.