necropolis vs potter's field

necropolis

noun
  • An ancient site used for burying the dead, particularly if consisting of elaborate grave monuments. 

  • A cemetery; especially a large one in or near a city. 

potter's field

noun
  • A public place where strangers, paupers, and criminals are buried. 

How often have the words necropolis and potter's field occurred in a corpus of books? (source: Google Ngram Viewer )