flatline vs pass the river

flatline

verb
  • To die. 

  • To stay at the same level, without development; or, to fall. 

  • To stop beating. 

  • To kill. 

noun
  • The disappearance of the rhythmic peaks displayed on a heart monitor. 

  • An unchanging state, as indicated in a graph of a variable over time. 

  • The disappearance of brain waves on an electroencephalogram. 

pass the river

verb
  • To die. 

How often have the words flatline and pass the river occurred in a corpus of books? (source: Google Ngram Viewer )