on offer vs unavailable

on offer

prep
  • For sale at a reduced price. 

  • Available to take; at hand; being offered or to be offered. 

unavailable

noun
  • A communist who operated underground and was therefore not available for a political leadership position. 

adj
  • Not available. 

How often have the words on offer and unavailable occurred in a corpus of books? (source: Google Ngram Viewer )