diction vs idiom

diction

noun
  • Enunciation, pronunciation. 

  • Choice and use of words, especially with regard to effective communication. 

  • The effectiveness and degree of clarity of word choice and expression. 

idiom

noun
  • A manner of speaking, a mode of expression peculiar to a language, language family, or group of people. 

  • An artistic style (for example, in art, architecture, or music); an instance of such a style. 

  • An established phrasal expression whose meaning may not be deducible from the literal meanings of its component words. 

  • A programming construct or phraseology that is characteristic of the language. 

  • A language or language variety; specifically, a restricted dialect used in a given historical period, context etc. 

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