Ipomoea vs sweet potato

Ipomoea

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 Ipomoea and sweet potato occurred in a corpus of books? (source: Google Ngram Viewer )