The graph is giving you an idea from emily's blog

If you have an missile barrage proc you can cast Arcane Missiles and use the proc. If you don't yet, you'll keep Arcane Blast fishing for WoTLK Gold a proc for missile barrage and as soon as you get the proc you can make Arcane Missiles. If you don't receive the proc, then at six casts, you would employ Arcane Missiles even though you do not have the Proc.

The graph is giving you an idea of just how powerful the DPS rotation is in terms of its man efficiency. We can see that the sweet spot is right between four six and four eight. And this is why private servers would always recommend just going to the forest axe and then using missile barrage. 

And if you don't have a barrage missile proc by that point continue using Arcane Blast and fishing for a missile barrage proc until you find the proc and then use Arcane Missiles because it's pretty cheap WoTLK Classic Gold likely that once you've stacked eight stacks, it will be possible to be able to get a missile barrage proc. so, typically, don't find yourself lucky enough to go over eight.