Does one say "allegory for" or "allegory of"?

Allegory of seems to be overwhelmingly favored over allegory for, per this Ngram. The Ngram utility does not allow my attempted comparison of “Pilgrim’s Progress is an allegory for/of,” but with the “Pilgrim’s” removed it showed no hits at all for the for option and some for the of option. Trials of “read/interpreted as an allegory of/for” also showed strong preference for of.

There are two ways to refer to an allegory. One is the title of it, which refers to the literal elements in the story, as "the allegory of the Cave" , as cited in the Wikipedia article you mentioned. The other way to refer to it is in regard to the things it symbolizes. In that sense, one uses "for" , e.g. It is an allegory for human self-centeredness. The syntax works the same for "parable".

Answer given by Brian Hitchcock (in the comments, but with a technical problem making it an answer).