St John's wort vs mugwort

St John's wort

noun
  • The plant prepared as a medication. 

  • Any of several plants, of the genus Hypericum, especially Hypericum perforatum, that have yellow flowers and traditionally ward off evil. 

mugwort

noun
  • Any of several aromatic plants of the genus Artemisia native to Europe and Asia. 

  • Artemisia vulgaris, traditionally used medicinally. 

How often have the words St John's wort and mugwort occurred in a corpus of books? (source: Google Ngram Viewer )