Can we get a damage prevented graph?

With every single spell that deals damage in the entire game, at the instant of actually taking the damage, you have two individual numbers. You have the total damage the ability would deal to a target at 0 resistance (i.e. Brand's Q would deal 240+65% AP damage upon impact), and how much damage you took (i.e. you have 30 MR, so you take 184.6 + 50% AP damage).

Regardless of how the damage is dealt, it's always individually registered; even DoTs are dealt in "ticks" or individual instances per second, half-second or quarter-second.

so attack speed slows don't count as damage mitigation? and damage reduction applied from other friendly champions counts as if it was your own damage reduction instead of their's?

Attack speed slows would actually be recorded more accurately in this system than in any system that tried to use math to estimate attack speed DPS reduction. This is because if an enemy champion does not hit you, it will not register damage. An attack speed reduction estimation would also produce misleading data, for example, if you apply 99% attack speed slows on all the enemy champions, but all the enemy champions are casters, it doesn't affect their total damage output on you in any way, so it doesn't make sense to say you provided damage mitigation in that case.

Personally, I don't see any problems with the 2nd point either - in similar logic to my first point, mitigation provided by allies increased your real, actual, gameplay-related mitigation, so it makes sense to record the mitigation there.

so attack speed slows don't count as damage mitigation?

Not really. Mitigation implies that they hit you, and you resisted some of it. An attack speed slow directly prevents them from attacking as frequently. If we were going to count AS slows, we'd also (logically) need to consider Silences in the same way, and that gets too theoretical.

and damage reduction applied from other friendly champions counts as if it was your own damage reduction instead of their's?

It would be less difficult to register that way. They applied it to you, so you 'borrowed' it. I'm sure, if they wanted, Riot could implement an algorithm specifically to calculate personal and allied DR separately, but the two main champions who would care are Kayle and Galio.