avow vs let cat out of the bag

avow

verb
  • To declare openly and boldly, as something believed to be right; to own, acknowledge or confess frankly. 

  • To bind or devote by a vow. 

  • To acknowledge and justify, as an act done. See avowry. 

let cat out of the bag

How often have the words avow and let cat out of the bag occurred in a corpus of books? (source: Google Ngram Viewer )