self-starter vs workhorse

self-starter

noun
  • A person who shows initiative and resourcefulness. 

  • A mechanism (usually one operated by electricity, compressed air, a spring, or an explosive gas), attached to an internal-combustion engine, as on an automobile, and used as a means of starting the engine without cranking it by hand. 

workhorse

noun
  • Anyone or anything that does a lot of work; something or someone who works consistently or regularly. 

  • A horse used primarily for manual labor; a draft horse. 

How often have the words self-starter and workhorse occurred in a corpus of books? (source: Google Ngram Viewer )