boon vs saving grace

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. 

saving grace

noun
  • A redeeming quality or factor. 

  • God's grace or blessing, enabling sinners to attain salvation. 

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