I changed it since you quoted it. It makes a big difference if you put capitals
https://books.google.com/ngrams/graph?content=Stonehenge%2CAvebury%2CSilbury&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2CStonehenge%3B%2Cc0%3B.t1%3B%2CAvebury%3B%2Cc0%3B.t1%3B%2CSilbury%3B%2Cc0