Unholy Heat checks your graveyard as it resolves to determine if it deals 2 or 6 damage. At that time, Unholy Heat isn't in the graveyard yet.

The card types that can appear in a graveyard are artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Kavu and Equipment are subtypes, not card types.