bishop's-wort vs selfheal

bishop's-wort

noun
  • fennel flower or devil-in-the-bush (Nigella damascena) 

  • wood betony (Stachys betonica, now Stachys officinalis) 

selfheal

noun
  • A small, herbaceous European plant with blue-violet flowers from any species of genus Prunella. 

  • Especially, Prunella vulgaris. 

How often have the words bishop's-wort and selfheal occurred in a corpus of books? (source: Google Ngram Viewer )