tone down vs weaken

tone down

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

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

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

weaken

verb
  • To make weaker or less strong. 

  • To become weaker or less strong. 

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