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 Accuracy 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.
Please note that Accuracy here is as a %, such that 2acc = 1acc%.
Wish more people would actually use these. Knowing how to use your gear is just as important as having it. After all, what good is your capped haste if you miss 20% of your hits.
According to this graph you need to have over 50% haste to overshadow 50% acc. To hit 50%, assuming you're not using hasso or any other job ability that provides haste, you need around 20% in gear, haste, and double march. This graph really shows the importance of accuracy and unless you have an extreme amount of haste accuracy normally wins.