go on vs is that so

go on

verb
  • To proceed. 

  • To use and adopt (information) in order to understand an issue, make a decision, etc. 

  • To happen (occur). 

  • To continue an action. 

  • To continue in extent. 

  • I think I've said enough now; I'm not sure I should go on. 

  • To talk about a subject frequently or at great length. 

intj
  • Expressing that the speaker can continue speaking without interruption from the listener. 

  • Expressing surprise, disbelief or incredulity. 

  • Expressing encouragement, see come on. 

is that so

phrase
  • Really? Is it true? 

How often have the words go on and is that so occurred in a corpus of books? (source: Google Ngram Viewer )