Damage taken charts should include the damage your Armor or MR absorbed also.

champion A: builds no armor and no MR and just survives of lifesteal and passive effects. loses the most health throughout the game.

Champion B: Build a ton of armor and MR but actually eats many more auto attacks and skills but doesnt take the most damage because of his armor and MR.

Champion B was the better tank but it will show as Champion A doing better.

only reason i rant is because damage taken is put into the grade you get at the end.