flat as pancake vs monotone

flat as pancake

monotone

noun
  • A piece of writing in one strain throughout. 

  • A single unvaried tone of speech or a sound. 

adj
  • Being, or having the salient properties of, a monotone function. 

  • Having a single unvaried pitch. 

verb
  • To speak in a monotone. 

How often have the words flat as pancake and monotone occurred in a corpus of books? (source: Google Ngram Viewer )