ennui vs monotony

ennui

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

verb
  • To make bored or listless; to weary. 

monotony

noun
  • Tedium as a result of repetition or a lack of variety. 

  • The property of a monotonic function. 

  • The quality of having an unvarying tone or pitch. 

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