plod vs step on

plod

verb
  • To trudge over or through. 

  • To walk or move slowly and heavily or laboriously (+ on, through, over). 

  • To toil; to drudge; especially, to study laboriously and patiently. 

noun
  • A slow or labored walk or other motion or activity. 

  • the police, police officers 

  • a police officer, especially a low-ranking one. 

step on

verb
  • Used other than figuratively or idiomatically: see step, on. 

  • to adulterate a drug for the purpose of increasing its bulk, and therefore its profit; to dilute, cut. 

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