imperturbability vs nonchalance

imperturbability

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

nonchalance

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

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