avow vs say so

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. 

say so

How often have the words avow and say so occurred in a corpus of books? (source: Google Ngram Viewer )