imperturbability vs self-possession

imperturbability

noun
  • A state of calm, unruffled self-assurance; aplomb, composure. 

self-possession

noun
  • calmness and composure, especially when under stress 

  • confidence in one's own powers 

How often have the words imperturbability and self-possession occurred in a corpus of books? (source: Google Ngram Viewer )