opus vs pièce de résistance

opus

noun
  • A work, especially of art. 

  • A work of music or set of works with a specified rank in an ordering of a composer's complete published works. 

pièce de résistance

noun
  • A masterpiece; the most memorable accomplishment of one’s career or lifetime. 

  • The chief dish at a dinner. 

How often have the words opus and pièce de résistance occurred in a corpus of books? (source: Google Ngram Viewer )