Not sure where the data comes from, but cultural context might play a role here. Per Chomsky (tinyurl.com/bn3fbtl), “libertarian” has a more conservative, pro-corporatist slant in the U.S. than it does elsewhere, where it’s often left-aligned.
The dataset, to my understanding, is all the books in the Google Books database.
Here’s the Chomsky, for those of you allergic to click-throughs:
The term libertarian as used in the US means something quite different from what it meant historically and still means in the rest of the world. Historically, the libertarian movement has been the anti-statist wing of the socialist movement. Socialist anarchism was libertarian socialism.
In the US, which is a society much more dominated by business, the term has a different meaning. It means eliminating or reducing state controls, mainly controls over private tyrannies. Libertarians in the US don’t say let’s get rid of corporations. It is a sort of ultra-rightism.
Whatever my thoughts on Chomsky — which, incidentally, are all derived from second hand observation — this doesn’t really jibe with my experience. Which, I should caution, has its own limits! I’ve never been involved with any socialist movements, but, as a non-American, I first encountered libertarianism in its American form, around 2000. I didn’t really notice non-Americans talking about libertarianism until Ron Paul came along. Newspapers in Australia who mention it still have to explain what it is.
So perhaps Chomsky is right that within certain socialist or anarchist subcultures, outside America, “libertarian” was a philosophy of the left. I wonder how accurately he, an American, can report on this non-American usage.
Most pertinently, however: Chomsky could be right and I could be wrong, and that would help explain the prevalence of the term in British English. At the same time though, I note that the real spike in British usage of the term coincided with the 1980s. It seems to me far more likely that 1980s British politics would help propagate a right wing definition of libertarianism rather than a left wing one.