boon vs grist for the mill

boon

noun
  • A good thing; a blessing or benefit; a thing to be thankful for. 

  • The woody portion of flax, separated from the fiber as refuse matter by retting, braking, and scutching. 

  • An unpaid service due by a tenant to his lord. 

adj
  • gay; merry; jovial; convivial. 

grist for the mill

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

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