e.g. vs that is to say

e.g.

adv

  • "for example" or "for the sake of an example": used to introduce an illustrative example or short list of examples. 

noun

  • An example. 

that is to say

adv

  • In other words. Used to introduce a clarification, simplification, or explanation 

How often have the words e.g. and that is to say occurred in a corpus of books? (source: Google Ngram Viewer )