expedience vs grist for the mill

expedience

noun
  • Something that is expedient. 

  • The quality of being fit or suitable to cause some desired end or the purpose intended; propriety or advisability under the particular circumstances of a case. 

  • Speed, haste or urgency. 

grist for the mill

noun
  • Something that is useful or that creates a favorable opportunity. 

How often have the words expedience and grist for the mill occurred in a corpus of books? (source: Google Ngram Viewer )