basically, with graves, u want to dash in and Q a single target trying to hit all 3 bullets, this will give a nice nuke. after Q u unleash some basic attacks because of ur dasha tt speed and then u go back.
keep doing this and try to stay far from melee champs, enlarging the gap between u and them with ur E
try contantly hitting something to recover ur dash cooldown and, if in trouble, u can use your W to blind the enemys tha are chasing you by landing it at you or a little bit above you, then dashing out, so they cant target or see you while inside the bubble.

with this build, graves becomes REALLY insane late game, criting over 900 with an awesome att speed, so u need to farm a lot during the game..
fortunatly, his farming is really good too
remember that ur Q does damage to ALL units that it passed through, so try being really close to the first minion and then, when they all stopped walking, land ur Q and basic attack the remaining minions.
u must keep ur CS over the other champs during the hole game and, when u buy the infinity + phantom, its pretty much GG

if u accomplished a nice early game and farming, at this point of the game u should be dealing so much damage that it is kinda ******ed...
at team fights, dont engange... thats TANKs job...let them do it, or ******s
if u engange
u will die
thats it...
try to stay back at teamfights and throw ur W in the middle of the fight, then, when everyones getting focused but not u
dash in and Q everyone, then ulti whene u can deal a lot of damage
focus the squishy mages case they will fal with 3 or 4 basic attacks
if u get focused, dash out and wait patiently
if u cant do that, try changing the last item into a starks fervor or another bloodthirsty
then u can tank everyone with ur lifesteal =P

PROS: graves is funny to play, kinda OP, deals a lot of damage, nice farm, great ingame skins (and he has a shotgun :3)

Cons: Graves is pretty much squishy at terms of being nuked y and ap caster like le blanc or lux, and he really cant get focused. Hes a little bit slow to me but his dash helps a lot.
if u getting pwned bynAP, consider buyng a banshee or an frozen mallet as ur last item =)