New Zealand yam vs sweet potato

New Zealand yam

noun
  • The oca, Oxalis tuberosa, a root vegetable introduced from South America. 

sweet potato

noun
  • The tuber of this plant cooked as a vegetable. 

  • An ocarina. 

  • A tropical perennial American vine, Ipomoea batatas, having a fleshy tuber. 

How often have the words New Zealand yam and sweet potato occurred in a corpus of books? (source: Google Ngram Viewer )