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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?

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I would think from it's literal etymology it should be valid for a graph. from the flat line on an electrocardiogram or electroencephalogram when the patient is dead – JoseK Sep 14 '11 at 9:34
up vote 0 down vote accepted

Google Books give some examples of using it to refer to graphs and other situations unrelated to death.

One example from Complexity, endogenous money and macroeconomic theory By Mark Setterfield, Basil J. Moore:

In any stationary state, the economy simply replicates itself year after year, with all its variables plotting as horizontal or 'flatline' graphs

and from Stable isotope ecology By Brian Fry

In essence, if we inherited a level playing field of isotope distributions as did the meteorites, emerging biology would create isotope lows and highs that would be easy to identify.

Viewed in this way, the flatline baseline from meteorite studies was an advantage, a good point for starting the planetary biology clock: where would life tick in, with its characteristic separation of the isotopes via fractionation?

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Well, "flatline" was used originally to denote a flat line on a graph -- Heart Monitor Equipment Graph. That's the thing that measures your heartbeat, and when there ceases to be any peaks, than you are dead. Hence, the meaning of "flatline" to mean dead:

Wiktionary:asystole; the absence of heart contractions
the disappearance of the rhythmic peaks displayed on a heart monitor

Dictionary.com: to die or be so near death that the display of one's vital signs on medical monitoring equipment shows a flat line rather than peaks and troughs

From what I've read, it seems that "flatline" is used nearly always in conjunction with death, your heart stopping, and the heart monitor. I don't think it's common usage to use it to refer to a flat line on a graph.

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+1 - The term is talking about heart monitors. Any other use is at least somewhat metaphorical. – T.E.D. Sep 14 '11 at 18:26

I searched here, to find this Q&A discussed the question I was going to pose.

enter image description here

This is from an Education Week article, aimed at teachers and those in education. The word flatline is used in the second sense, the one you were questioning.

Usage is fluid. If enough people start using it in this sense, it will be less awkward and stop referring only to dying. Although I admit it struck me as odd, the fact that the origin is from a graph flatlining, the usage makes sense.

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