allegory vs backstory

allegory

noun
  • A narrative in which a character, place, or event is used to deliver a broader message about real-world issues and occurrences. 

  • A symbolic representation which can be interpreted to reveal a hidden meaning, usually a moral or political one. 

  • A picture, book, or other form of communication using such representation. 

  • A category that retains some of the structure of the category of binary relations between sets, representing a high-level generalisation of that category. 

backstory

noun
  • The previous life and experiences of a person, especially a character in a dramatic work, but also a real-life person. 

  • A fictitious account of a person's history designed to conceal their actual history and motives. 

  • A prequel. 

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