incunabulum vs protograph

incunabulum

noun
  • A book, single sheet, or image that was printed before the year 1501 in Europe. 

  • The cradle, birthplace, or origin of something. 

protograph

noun
  • The original manuscript of a given text, from which all further copies derive. 

  • A projected graph. 

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