@ossobuco,
Go on, I dare you...
Basically it's a graphical representation of the schisms of christianity (coloured branches above) and the unschismed straight grey line of atheism (below).
Personally I think it's a tad pointless because it suggests that there is an atheist manifesto that you need to follow to be an atheist. Which is bollocks. I think anti-atheist religionists like to paint atheism as a religion because they know how to fight another religion. A bit like the war on terror invading countries, actually.
The chart isn't pro-atheist, it's anti-religion. (and not anti-theist in the general sense).