Don't get me wrong. This Wikipedia graph carries a lot of information -- I use it all the time -- but there's a counter-intuitive quality about it that bothers me and I have no idea how to fix it.
The problem is that changes in the x-axis mean more or less the opposite of changes in the y-axis. Smaller intervals between new laws and larger jumps in duration both indicate increased copyright protection.
This wouldn't be a problem if we were plotting these with a line -- with lines we're used to thinking in terms of slope -- but here the natural impulse is to think in terms of area, thus making the 1976 and 1998 laws look like fairly minor changes.
One more complaint, all but one of the states had copyright laws before 1790 so the law passed that year was more of a formalization than an extension. For most of the states, there was a period of almost fifty years without a major extension. The twenty-two year interval before the 1998 act really was exceptionally short.