I think the big spike on the graph indicates 1974 actually. My dad who was a cop at the time remembers it better than I do, he was very busy that evening. We were living in northern Alabama then (we moved to Florida when I was still quite young), and the old man still shudders when he remembers that outbreak, which is regarded as the worst in US history, and plainly evident by the graph.
Check this link out for the info:
http://en.wikipedia.org/wiki/Super_Outbreak
EDIT: I think, in response to your original post, a lot of people are easily persuaded to believe that these current events is indicative of a greater global climate shift taking place, but the reality of it is that it is a relative perception - when in earlier days, and especially in more rural areas, things like this often went unreported outside of their local regions - with today's close-knit telecommunications technology, word can spread instantly through a hundred different resources; and the media hype only fuels speculation as to the cause - to the average observer it may appear that s**t has lately hit the proverbial fan, when in actuality it's just the same old weather patterns we've always seen, we're just being made more aware of it these days.