I was looking at this ngram which features a flat line meaning absolutely no usage of the word I was looking for. I thought about describing it in these terms:
The Google ngram clearly flatlines
But then, I checked the New Oxford American Dictionary, which says:
flatline (verb intrans., informal)
[of a person] die.
So, can flatline be used to denote an actual flat line on a graph, or is it only used figuratively?