head-on vs in line

head-on

adj
  • Direct, abrupt, blunt or unequivocal; not prevaricating. 

  • Of a collision, from the front or in the direction of motion. 

adv
  • With the front of a vehicle. 

  • With direct confrontation. 

noun
  • A collision from the front. 

in line

adj
  • Suitable or appropriate; keeping with expectations, norms, ideals, or rules. 

  • Positioned in a straight line. 

  • assuming a position in the future. 

adv
  • one behind another; in a queue 

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