R-rated vs XXX

R-rated

adj
  • Not suitable for children, especially due to involving excessive sexual, violent, or rude content 

  • Of a motion picture, having an "R rating" from one more motion picture rating authorities, typically meaning it is suitable only for adults. 

  • Of a motion picture, having an "R rating" from the Motion Picture Association (MPA), meaning it is suitable only for adults. 

XXX

adj
  • Designating extreme or hardcore pornography. 

  • Extra strong; very high quality. 

noun
  • Triple X syndrome. 

symbol
  • An indicator of hazard or poison. 

  • Traditional brewer's mark for barrels of extra strong beer—extended to other alcoholic beverages and other products having an extra strong dimension (varying from laundry soap to pornography). 

verb
  • To delete; to cross out, especially to conceal or suggest vulgar language. 

How often have the words R-rated and XXX occurred in a corpus of books? (source: Google Ngram Viewer )