The explanation on the main page says that the diagrams show which recordings are overdriven. I don't really understand them but when you look at the diagrams for manowar, metallica, and nightwish you'll see that they hit the top and bottom lines. On the other hand in the diagrams for classical music (Mozart, Pergolesi, Mendelssohn) the lines end on the left and right sides or at most hit the corners. Since I doubt that the classical recordings are overdriven going through the roof is probably no good.
As for Opeth, the BWP curves aim straight for the top and bottom lines but bends just in time to smoothly hit the corners.
The Damnation diagram looks like completely overdriven, but then again the heading tells that it is for an album of Immortal. So, maybe the diagram is really not Opeth.