pin-up vs pinup

pin-up

noun
  • A photograph, printed in a magazine or other publication, of a sexually attractive person (often nude or provocatively dressed), and intended to be removed and pinned up on a wall. 

  • The person so depicted. 

  • Figurehead, person who represents an idea, cause etc. 

pinup

How often have the words pin-up and pinup occurred in a corpus of books? (source: Google Ngram Viewer )