fall on vs ought to

fall on

verb
  • To be assigned to; to acquire a new responsibility, duty or burden. 

  • To experience; to suffer; to fall upon. 

  • To occur on a particular day. 

  • Of the eyes or gaze: to alight upon. 

ought to

verb
  • Indicating duty or obligation. 

  • Indicating advisability or prudence. 

  • Indicating desirability. 

  • Indicating likelihood or probability. 

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