The problem with graphs like that is that they are so narrowly focused. Yes, they correctly show the 300 ppm line, and the 400 ppm line, but they don't go all the way up. If you're going to show parts per million, then show from zero to a million, it's simple. If you have a graph the size of a football field (100 yards long)divided into a million parts then the 300 ppm line is at the very first inch at the bottom. 400 ppm means that, over the football field sized graph, the line has risen 2/5 of an inch more. It seems less dramatic when you include the whole picture and almost invisible if you place the whole graph on a sheet of paper.