Interesting, although the smoothing can be a bit misleading. As I expected, "open source" does not exist before 1998, when Eric Raymond, Bruce Perens, et al coined the term:

https://books.google.com/ngrams/graph?content=open+source&ca...

However, with data smoothing, it makes it appear is if the term existed before 1998:

https://books.google.com/ngrams/graph?content=open+source&ca...