cover up vs pencil in

cover up

verb
  • To conceal or disguise. 

  • To clothe. 

  • To put covers over oneself in bed. 

pencil in

verb
  • to provisionally schedule 

  • to fill in or write in using pencil 

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