When you enter phrases into the Google Books Ngram Viewer, it displays a graph showing how those phrases have occurred in a corpus of books over the selected years.
The default corpus of books being:
The "Google Million". All are in English with dates ranging from 1500 to 2008. No more than about 6000 books were chosen from any one year, which means that all of the scanned books from early years are present, and books from later years are randomly sampled.
I tried it on a few things I knew (for instance the William Blake revival) and it accurately reflected them.
Here are some more I looked at. It's very quick to make the graphs. Note: it's case sensitive, and you need to set the year range.
Gawain (there's the discovery of the manuscript in the early 19th c)
some political parties (unfortunately Labour capitalised also means the abstract concept, and WWII Ministry so artificially high)
feminism (it's coming back ladies)