Google Ngram Viewer - pain term trends

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 (e.g., "British English", "English Fiction", "French") over the selected years.

http://ngrams.googlelabs.com/graph?content=fibromyalgia,+chronic+pain&year_start=1800&year_end=2000&corpus=0&smoothing=3