oligarch vs plutocrat

oligarch

noun
  • A very wealthy business owner who wields political power. 

  • A member of an oligarchy; someone who is part of a small group that runs a country. 

  • A protoplanet formed during oligarchic accretion. 

plutocrat

noun
  • Someone who rules by virtue of his or her wealth. 

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