anybody's guess vs question mark

anybody's guess

noun
  • Synonym of anyone's guess 

question mark

noun
  • An enigmatic, inscrutable, or mysterious person or thing; an enigma, a riddle. 

  • The punctuation mark "?", used at the end of a sentence to indicate a question. 

  • A state of doubt or uncertainty. 

  • Polygonia interrogationis, a North American nymphalid butterfly with a silver mark on the underside of its hindwing resembling a question mark (sense 1). 

How often have the words anybody's guess and question mark occurred in a corpus of books? (source: Google Ngram Viewer )