herb tea vs tisane

herb tea

noun
  • A beverage (a tisane) made by steeping herbs (sometimes also includes other plants, such as spices), other than tea leaves, in hot water. Technically not a tea because it does not contain tea leaves. 

tisane

noun
  • A medicinal drink, originally made from barley soaked in water. 

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