bachelor's button vs blue bonnet

bachelor's button

noun
  • Any one of a number of plants with daisylike, button shaped flowers of the genus Centaurea, especially the species Centaurea cyanus. 

  • A button which may be attached to clothing without resorting to needle and thread. 

blue bonnet

noun
  • Any of several blue flowering plants, especially cornflower, Centaurea cyanus, and plants of the genus Scabiosa. 

  • A blue Australian parrot, especially Northiella haematogaster. 

  • A traditional flat Scottish hat made of blue wool; a blue tam-o'-shanter. 

  • Someone who wears such a hat; a Scotsman, especially a Scottish soldier. 

  • Any of several North American species of Lupinus common in the southwestern US, especially Lupinus texensis and Lupinus subcarnosus. 

How often have the words bachelor's button and blue bonnet occurred in a corpus of books? (source: Google Ngram Viewer )