Chinese grapefruit vs grapefruit

Chinese grapefruit

noun
  • Synonym of pomelo (“SE-Asian citrus fruit”) 

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 Chinese grapefruit and grapefruit occurred in a corpus of books? (source: Google Ngram Viewer )