ennui vs unhappiness

ennui

noun
  • A gripping listlessness or melancholia caused by boredom; depression. 

verb
  • To make bored or listless; to weary. 

unhappiness

noun
  • The feeling of not being happy 

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