ipomoea vs sweet potato

ipomoea

noun
  • Any of various twining plants of the genus Ipomoea with showy monopetalous flowers, including the morning glory, the sweet potato, and the cypress vine. 

sweet potato

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

  • The tuber of this plant cooked as a vegetable. 

  • An ocarina. 

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