caliph vs oligarch

caliph

noun
  • The political leader of the Muslim world; the successor of the prophet Muhammad's political authority. 

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. 

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