picket line vs sit-down

picket line

noun
  • A boundary guarded by a picket (unit of soldiers). 

  • A barrier or fortification formed by pickets; a stockade. 

  • A boundary created by workers participating in a strike, generally at the workplace entrance, which other workers are asked not to pass. 

  • A line or rope held by one or many pickets, chiefly one used for tethering horses. 

sit-down

noun
  • A sit-in, a protest of civil disobedience by people sitting and refusing to move. 

  • An act of sitting down, especially with other people in some form of social exchange. 

adj
  • Intended to be done, used, consumed etc. while sitting. 

How often have the words picket line and sit-down occurred in a corpus of books? (source: Google Ngram Viewer )