chat up vs come on to

chat up

verb
  • To talk to (someone) in a friendly, open, or casual manner, sometimes also in a charming or affected manner, usually to curry favor, and sometimes flirtatiously with the intention of establishing a romantic or sexual encounter or relationship with that person. 

come on to

verb
  • To make a romantic or sexual advance to. 

  • To start to. 

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