heir vs next in line

heir

verb
  • To inherit. 

noun
  • A successor in a role, representing continuity with the predecessor. 

  • Someone who inherits, or is designated to inherit, the property of another. 

  • One who inherits, or has been designated to inherit, a hereditary title or office. 

next in line

How often have the words heir and next in line occurred in a corpus of books? (source: Google Ngram Viewer )