expediency vs ghost writer

expediency

noun
  • An expedient. 

  • The quality of being fit or suitable to effect some desired end or the purpose intended; suitability for particular circumstance or situation. 

  • Pursuit of the course of action that brings the desired effect even if it is unjust or unprincipled. 

ghost writer

How often have the words expediency and ghost writer occurred in a corpus of books? (source: Google Ngram Viewer )