The source data was Google's web n-grams database which is released by Google under the Creative Commons Attribution 3.0 Unported licence.

I created the two graphical curves with Google's ngram viewer copie d'archive sur Wayback Machine which generates a bitmap output. With Inkscape, I extracted the two curves from the bitmap and converted them to vector-objects, then I manually re-drew the grid and legends.

I set the ngram viewer to show the incidence of the words "laſt" and "last" in all English documents (including British and American) from 1700 to 1900. I actually entered the words "laft" (with a small-F) and "last" because Google's OCR technology misread the long-s as an "f". This has been corrected by Google since the creation of this picture, so to be able to make a distinction between the two kinds of "s", one has to select one of the 2009 corpora.

The ngram viewer allows the user to examine a sample of documents that were referenced by the search with the target words highlighted. I saw that the apparent minor peaks of short-s usage during 1700-1800 were largely not due to actual short-s usage, but due to the occasional use of a different OCR technology that was able to recognize the long-s as an "s" rather than an "f". This was particularly noticeable when the ngram viewer was set to look at American documents only, which gave unusably uneven curves from 1700-1800, partly because there are not enough American documents from that period to form smooth curves, and because of the usage of the better OCR on about one-third of the American documents from that period.

Data from Google used to create this diagram:

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