plateau vs stagnate

plateau

verb
  • (of a varying quantity) To reach a stable level after a period of increase; to level off. 

noun
  • A comparatively stable level after a period of increase. 

  • A notable level of attainment or achievement. 

  • A largely level expanse of land at a high elevation; tableland. 

stagnate

verb
  • To cease to develop, advance, or change; to become idle. 

  • To cease to flow or run. 

  • To be or become foul from standing. 

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