It's a histogram. Each color is a different simulated physical process: they can all happen in particle collisions, so the sum of all of them should add up to the data the experiment takes. The data isn't shown here because it hasn't been taken yet: this is an extrapolation to a future dataset. And the dotted lines are some hypothetical signal.
The area occupied by each color is basically meaningless, though, because of the logarithmic y-scale. It always looks like there's way more of whatever you put on the bottom. And obviously you can grow it without bound: if you move the lower y-limit to 1e-20 you'll have the whole plot dominated by whatever is on the bottom.
For the record I think it's a terrible convention, it just somehow became standard in some fields.
The area occupied by each color is basically meaningless, though, because of the logarithmic y-scale. It always looks like there's way more of whatever you put on the bottom. And obviously you can grow it without bound: if you move the lower y-limit to 1e-20 you'll have the whole plot dominated by whatever is on the bottom.
For the record I think it's a terrible convention, it just somehow became standard in some fields.