apology vs rationale

apology

noun
  • A formal justification, defence. 

  • An expression of remorse or regret for having said or done something that harmed another: an instance of apologizing (saying that one is sorry). 

  • Anything provided as a substitute; a makeshift. 

rationale

noun
  • A justification or rationalization for something. 

  • An explanation of the basis or fundamental reasons for something. 

  • A liturgical vestment worn by some Christian bishops of various denominations. 

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