ignite vs set afire

ignite

verb
  • to set fire to (something), to light (something) 

  • to commence burning. 

  • to spark off (something), to trigger 

  • To subject to the action of intense heat; to heat strongly; often said of incombustible or infusible substances. 

set afire

verb
  • Synonym of set on fire 

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