sweet potato vs truffle

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. 

truffle

noun
  • Any of various edible fungi, of the genus Tuber, that grow in the soil in southern Europe; the earthnut. 

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