I've been playing with these. Use of "millenium" is highest right around 1980, but references diminish as the millenium approached. And "countenance", one of my favorite verbs, is at its high point in the 1830s. It then begins a long slow slide to the present. But "renege" is the opposite; it begins its climb in the 1930s and, except for a small decline in the mid-1990s, keeps on moving up. And both God and the devil are sinking like stones.