Thats an option in Xperl to show a basic threat meter. The further to the right those green bars go the more threat each shaman is generating Once a green thing goes all the way to the right it turns red when the shaman pulls aggro.

Its very basic (not as advanced as Omen) but I find it much easier to follow since I'm already looking over there anyway and I don't have to install it on my 4 clones.