work to rule vs work-to-rule

work to rule

verb
  • To work slowly by strictly following all work rules, usually in a work-to-rule job action, as part of unionized labor protest. 

work-to-rule

noun
  • A labor protest in which employees do only the minimum work required by the rules of a workplace, following safety or other regulations to the letter in order to cause a slowdown. 

adj
  • Of or pertaining to such a protest. 

How often have the words work to rule and work-to-rule occurred in a corpus of books? (source: Google Ngram Viewer )