The reason I go 21/0/9 is that Graves has tank from his passive, and the extra mana regen allows you to harass more with your buckshot. Thanks for the feedback though, it's my first guide so I figured I would just go with the basics.

Maybe so, but IE + PD = 55% Crit Chance, so Graves already has the ability to Crit every 2 hits.

yea, 21/9/0 is better for graves.
Your build is okay, but you rely on crits. I honestly prefer BT PD on graves. his passive provides amazing sustainability with the lifesteal and his spells will deal more damage with BT than with IE.

What ^ said about looking at jhoihoi's guide: yours is ok, but lacks more info and, more important, to be worked aesthetically so it doesn't tire the reader. You have good tips tho.

Also, I think starting just with AD runes migh not be enough; you should have at least an armour penetration set.

A good thing to do on guides for carries is teaching how to solo and how to interact with different supports, which best suits your carry, etc.

Well made guide overall, I would maybe get the boots a bit earlier and go 21-9-0 on masteries, but what the guide really needs is some coding.
I'd recommend that you take a look at jhoihoi's guide how to make guide