trilogy vs triumvirate

trilogy

noun
  • A set of three works of art that are connected, and that can be seen either as a single work or as three individual works. They are commonly found in literature, film, or video games. 

triumvirate

noun
  • An official group of three people, especially a ruling council of three men and particularly (historical) two such councils in Roman history. 

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