purple yam vs taro

purple yam

noun
  • A yam used in a variety of desserts, Dioscorea alata. 

taro

noun
  • Food from a taro plant. 

  • Any of several other species with similar corms and growth habit in Colocasia, Alocasia etc. 

  • Colocasia esculenta, raised as a food primarily for its corm, which distantly resembles potato. 

How often have the words purple yam and taro occurred in a corpus of books? (source: Google Ngram Viewer )