implication vs undertone

implication

noun
  • An implying, or that which is implied, but not expressed; an inference, or something which may fairly be understood, though not expressed in words. 

  • The act of implicating. 

  • The state of being implicated. 

  • The connective in propositional calculus that, when joining two predicates A and B in that order, has the meaning "if A is true, then B is true". 

  • Logical consequence. 

  • A possible effect or result of a decision or action. 

undertone

noun
  • An implicit message perceived subtly alongside, but not detracting noticeably from, the explicit message conveyed in or by a book, film, verbal dialogue or similar (contrast with overtone); an undercurrent. 

  • A pale colour, or one seen underneath another colour. 

  • A low state of the physical faculties. 

  • An auditory tone of low pitch or volume. 

verb
  • To accompany as an undertone. 

  • To say or speak in an undertone. 

  • To present as less important, noticeable or prominent. 

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