nonchalance vs self-possession

nonchalance

noun
  • Indifference, unconcern; carelessness; coolness; disregard, detachment. 

self-possession

noun
  • calmness and composure, especially when under stress 

  • confidence in one's own powers 

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