on offer vs on tap

on offer

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

  • For sale at a reduced price. 

on tap

prep
  • On an agenda, planned, expected. 

  • Available directly from the barrel by way of a tap. 

  • Readily available; ready for use in whatever quantity is desired. 

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