I've only just discovered Google's Ngram Viewer http://ngrams.googlelabs.com/
I believe it shows how often particular terms appeared in a shitload of books, by publication date of the books. I'm no expert - do check this for yourself. I've found that the search limits can be altered to search through stuff as recent as 2008.
I have done some searches on the terms "synaesthesia" and "synesthesia". The results have been interesting, but probably not a firm basis for conclusions. I found that both graphs are a long, steady increase from roughly around the beginning of the last century, but are also quite different for each of these terms - one the American term "synesthesia" with the simplified spelling, the other with the quaint old spelling "synaesthesia" which is associated with British writers and researchers. Another point of interest is that I found little evidence of a resurgence of interest in synaesthesia as a subject of study in the 1980s, for either of the spellings. A big upswing in interest appears to have happened much later, in the mid 1990s in the UK, spreading later to the US, so I can only guess that British researchers or authors deserve the credit for the most recent upswing in interest in this scientific subject.
I did find a definite peak in the graph for "synaesthesia" (but not for the US spelling) around 1960, probably something to do with monkeying around in the UK with LSD for fun or under the banner of "therapy". Perhaps Syd Barrett and company deserve some "credit" here. I also found a decline in the graph after the year 2001 for the term "synaesthesia", not sure why. I have read some online comments on a recent UK media story about synaesthesia in which readers complained that they are bored with synaesthesia as a subject, so perhaps synaesthesia has not been the flavour of the month for a long time in the UK.
Many things need to be kept in mind about my searches using this tool - I don't think the viewer covers journal papers or other non-book publications, so I don't think it should be seen as a general reflection of scientific research. One also needs to remember that synaesthesia has been researched and written about under many different terms over a long period of history (see my article cited below), and searches on the modern terms will not retrieve those items. Sir Francis Galton was an early synaesthesia researcher, but I don't think he ever used the term "synaesthesia" (correct me if I am wrong). Boris Sidis, the psychologist and researcher father of the child prodigy William James Sidis, was also an early writer on the subject of synaesthesia, but he used the term "secondary sensation" in his writing.
Words that have been used as terms for synaesthesia in popular, scientific or clinical literature