citron vs grapefruit

citron

noun
  • A small citrus tree, Citrus medica. 

  • The fruit of a citron tree. 

  • A greenish yellow colour. 

  • The candied rind of the citron fruit. 

adj
  • Of a greenish yellow colour. 

grapefruit

noun
  • The tree of the species Citrus paradisi, a hybrid of pomelo (Citrus maxima) and sweet orange. 

  • The large spherical tart fruit produced by this tree. 

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