citronella vs grass oil

citronella

noun
  • An essential oil obtained from this plant, often used as an insect repellent. 

  • A tropical Asian grass, Cymbopogon nardus, that has citrus-scented leaves. 

grass oil

noun
  • A fragrant, volatile essential oil, obtained in India from grasses of the genus Cymbopogon, etc., and used in perfumery. 

  • Any of various volatile oils derived from widely different plants. 

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