front line vs vanguard

front line

noun
  • attack, collectively the attackers or forwards. 

  • A low level. 

  • A site of a conflict, effort, or controversial matter of any kind. 

  • A front, or a boundary between opposing positions. 

  • The site of interaction with outsiders, such as customers. 

vanguard

noun
  • The person(s) at the forefront of any group or movement. 

  • The leading units at the front of an army or fleet. 

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