slave-driver vs taskmaster

slave-driver

noun
  • An employer who demands excessive amounts of work from employees. 

  • An overseer of slaves at work; a person who puts slaves to work. 

taskmaster

noun
  • Someone who supervises workers, especially one who imposes hard or burdensome work. 

  • A source of hard work or responsibility. 

How often have the words slave-driver and taskmaster occurred in a corpus of books? (source: Google Ngram Viewer )