highway robbery vs undue

highway robbery

noun
  • The act of robbing a traveler on a public road. 

  • Said of excessive or exorbitant prices. 

undue

adj
  • That which ought not to be done; illegal; unjustified. 

  • Not owing or payable. 

  • Excessive; going beyond that what is natural or sufficient. 

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