I'm no math whiz, so take this with a grain of salt, but I think it's basically showing how often they had a certain goals versus goals expected goals per game.
Campbell's being very steep and below zero means he often allows more goals than expected while Skinner has a much more level line that peaks above zero which means he's been consistently above average and has more great games than he's had bad ones. Campbell, on the other hand, has almost no great games to make up for all the eggs he's laid.
edit: The graph in the article makes a little more sense.