sell down the river vs stab in the back

sell down the river

verb

  • To betray, especially in a manner which causes serious difficulty for the one betrayed. 

stab in the back

verb

  • To betray (somebody). 

noun

  • An act of betrayal or treachery 

How often have the words sell down the river and stab in the back occurred in a corpus of books? (source: Google Ngram Viewer )