peso vs piece of eight

peso

noun
  • A former unit of currency in Spain and Spain's colonies, worth 8 reales; the Spanish dollar. 

  • The circulating currency of various Spanish-speaking American countries (Argentina, Chile, Colombia, Cuba, the Dominican Republic, Mexico, and Uruguay) and the Philippines. 

piece of eight

noun
  • The peso, a silver coin that was minted in Spain and its colonies from the end of 15th century, equal to eight reales (hence the name). 

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