customer vs travelling salesman

customer

noun
  • A patron, a client; one who purchases or receives a product or service from a business or merchant, or intends to do so. 

  • A native official who exacted customs duties. 

  • A person, especially one engaging in some sort of interaction with others. 

travelling salesman

noun
  • A man who travels, often within a certain territory, soliciting orders or selling goods. 

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