Add selection.context.

[mbostock]

I’m not sure about this API. It’s nice to have something that can capture the context of a selection.each (or selection.attr, selection.data, etc.). But it’s a bit weird that you have to pass in arguments and then have the selection recompute the ancestors.

Also, sometimes you might want to access the current group or the parent node within a callback. The selection.context can determine those things, but doesn’t expose them.

Also, within a selection.each (or most other selection methods) callback, you don’t have easy access to the current selection because the this context is set to the current element. So maybe an approach like d3.event is more suitable? Like, say, d3.context() could be a magic method that only works within selection.each, and returns the context for the current selection.

[mbostock]

Also, it might not be necessary to expose this functionality if transition events went through the selection.dispatch mechanism… though it does seem reasonable to want access to ancestors within a callback.