From my life in Britain the phrase always brings to mind its usage in Parliament in 1979 (http://en.wikipedia.org/wiki/1979_vote_of_no_confidence_in_t...) which seems to have been the first time many of us encountered it.

It comes from Callaghan's speech (as quoted in the article):

"We can truly say that once the Leader of the Opposition discovered what the Liberals and the SNP would do, she found the courage of their convictions. So, tonight, the Conservative Party, which wants the Act repealed and opposes even devolution, will march through the Lobby with the SNP, which wants independence for Scotland, and with the Liberals, who want to keep the Act. What a massive display of unsullied principle! The minority parties have walked into a trap. If they win, there will be a general election. I am told that the current joke going around the House is that it is the first time in recorded history that turkeys have been known to vote for an early Christmas."

That explanation fits well with the n-grams results. Thank you.

The plural of anecdote is not data, but as a Brit, I am familiar with the phrase.