tone down vs water something down

tone down

verb
  • To make a television program, piece of writing, etc. less offensive and so more suitable for a family audience. 

  • To moderate or relax; to diminish or weaken the striking characteristics of; to soften. 

  • To relax; to make quieter or less obtrusive; to make milder. 

water something down

How often have the words tone down and water something down occurred in a corpus of books? (source: Google Ngram Viewer )