sign on vs subscribe

sign on

verb
  • To commit oneself, as to a project, a goal, an organization, a cause. 

  • To begin broadcasting a radio or television signal, usually at the beginning of a broadcasting day and after being off the air for several hours. 

  • To join something, after signing. 

  • To apply to receive unemployment benefits. 

  • To log on; to start using a computer, radio, etc., or to start talking. 

noun
  • The time of day when a radio or television station begins broadcasting, usually after being off the air for several hours. 

subscribe

verb
  • To contribute or promise to contribute money to a common fund. 

  • To sign up to have copies of a publication, such as a newspaper or a magazine, delivered for a period of time. 

  • To agree to buy shares in a company. 

  • To pay money to be a member of an organization. 

  • To indicate interest in the communications made by a person or organization. 

  • To believe or agree with a theory or an idea (used with to). 

  • To pay for the provision of a service, such as Internet access or a cell phone plan. 

  • To register for notifications about an event or similar. 

  • To sign; to mark with one's signature as a token of consent or attestation. 

  • To promise to give, by writing one's name with the amount. 

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