If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
I am not sure which data you are looking at.
My link shows the percentage of people who self-identify as Evangelicals. The data starts in 1991 and ends in 2005. The first values (1991-1993) are 41%, 42%, 46%, 44%, 43%, and the last values (2004-2005) are 42%, 39%, 42%, 47%, 40%.
I see no trend.
Your link shows the percentage of people who answer three proxy questions. The data starts in 1976 and ends in 2005. Over that time period one question goes up (47% to 52%), one goes down (38% to 32%) and the third goes up as well (35% to 48%). Do note that the survey says "When looking at the percentage of Americans who say yes to all three of these questions, slightly more than one in five (22%) American adults could be considered evangelical" and that's about *half* of the number of people who self-identify as such.
Given all this, I see no evidence that the mind share of the Evangelicals in the US is increasing.
The proxy I am specifically looking at for evangelical Christianity is people who claim to have spread the "good news" about Jesus to someone. In other words, asking people whether they themselves have evangelized (the data on this is the fairly clear 47% to 52% upward trend). To me, it makes a lot of sense to call someone an Evangelical Christian if they have in fact evangelized for Christianity. And if we disagree on that definition, then there is really nothing more I can say.