picket line vs sleep-in

picket line

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

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

  • 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. 

sleep-in

noun
  • An instance of sleeping in, sleeping late, sleeping until one feels fully refreshed rather than rising at a prespecified time. 

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