on offer vs piece of cake

on offer

prep
  • For sale at a reduced price. 

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

piece of cake

noun
  • Used other than figuratively or idiomatically: see piece, cake. 

  • A job, task or other activity that is pleasant – or, by extension, easy or simple. 

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