Fix `this` context for dashboard accessors

At the moment, dashboard accessors are given a `this` context for the current DOM element they are being accessed for, when they should be getting the current dashboard DOM element they are being accessed for.
Since js doesn't enforce specifying all the parameters, its only inevitable that I'd by mistake leave some out. I'd also like to follow d3 patterns wherever possible for sapphire, and only using the arguments needed is one of them. If people are familiar with d3 or the docs, they would know that `i` is available.

I know this is mostly a style thing, but I'd really like to keep it this way, if that's alright.