French bean vs string bean

French bean

noun

  • haricot bean, kidney bean. 

string bean

noun

  • A common bean (Phaseolus vulgaris) 

  • A hyacinth bean (Lablab purpureus) 

  • Any long, slender green bean. 

  • A tall and thin person. 

  • An immature runner bean (Phaseolus coccineus) 

  • A yardlong bean (Vigna unguiculata subsp. sesquipedalis) 

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