At that point when you've got the missile barrage proc you WoTLK Gold would use Arcane Missiles to consume the proc. If not then you'll continue casting Arcane Blast fishing for a proc for missile barrage and as soon as you get the proc, you will make Arcane Missiles. And if you don't get the proc, after six castings, it is possible to make use of Arcane Missiles, even if you do not have the Proc.
In essence, this graph is giving you an idea of the level the DPS rotation is in relation to its man efficiency. We can see that the sweet spot is right between four six and eight. And this is why private servers always recommend taking a for a forest axe using missile barrage.
And if you don't have a barrage missile proc, to keep casting Arcane Blast and fishing for a missile barrage proc until you find the proc and then using Arcane Missiles because it's pretty likely that, by the time you've got eight stacks you will have a missile barrage cheap WoTLK Classic Gold relic and therefore, generally don't become that unlucky to have to go beyond eight.
