sobriquet vs tropical sweet potato

sobriquet

noun
  • A familiar name for a person or thing; a nickname (sometimes assumed by the person, but often given by others), that is descriptive. 

tropical sweet potato

noun
  • Synonym of name (yam) 

  • Synonym of white sweet potato 

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