fairy story vs figment of one's imagination

fairy story

noun
  • Synonym of fairy tale (“folktale with fantasy characters”) 

  • Synonym of fairy tale (“story presented as true that is not believable”) 

figment of one's imagination

noun
  • A product of the imagination. 

How often have the words fairy story and figment of one's imagination occurred in a corpus of books? (source: Google Ngram Viewer )