citrus fruit vs clementine

citrus fruit

noun
  • Any of the fruits in the common genus Citrus of flowering plants in the family Rutaceae. 

clementine

noun
  • A type of small, sweet orange, the result of a cross between a tangerine and Seville orange. 

How often have the words citrus fruit and clementine occurred in a corpus of books? (source: Google Ngram Viewer )