Just posting this since I end up linking it a lot and putting it here gives people somewhere to refer to if they need it.
About the y-axis since it causes a lot of confusion:
The y-axis represents the improvement in damage as a %! It is a not a positive/negative thing. The graph is not suggesting that Double Attack actually reduces your damage as you add more or anything like that, just that each point you add increases your damage by less than the previous point added.