obfuscate vs pencil in

obfuscate

verb
  • To deliberately make more confusing in order to conceal the truth. 

  • To alter code while preserving its behavior but concealing its structure and intent. 

  • Before leaving the scene, the murderer set a fire in order to obfuscate any evidence of their identity. 

  • To make dark; to overshadow. 

pencil in

verb
  • to provisionally schedule 

  • to fill in or write in using pencil 

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