go to vs sit in on

go to

verb
  • To attend classes at a school as a student. 

  • To attend an event or a sight. 

  • Used other than figuratively or idiomatically: see go, to. 

  • To tend to support. 

noun
  • The branching construct GOTO. 

sit in on

verb
  • To attend a private meeting or class without participating. 

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