hierarchify

[mbostock]

This helper for d3.stratify imputes internal nodes using the specified path function, which returns slash-separated ids (as per a typical UNIX-y file system). Fixes #33.

Now that JavaScript has all sorts of niceties for destructuring and optional arguments, I’d also like to start moving away from the D3 “getter-setter” pattern in favor of options objects. For example, instead of:

const root = d3.stratify().id(d => d.id).parentId(d => d.parentId)(data);

You’d say:

const root = d3.stratify(data, {id: d => d.id, parentId: d => d.parentId));

This allows D3 classes to favor immutability, and makes it easier to share options via the spread operator, too. I’ve done that internally here for d3.stratify, but the new option-based API is not exposed externally. I figure we can explore changing the internal API first, although I’m happy to revert this part if you think it’s too aggressive.

I originally thought about this being a path option for d3.stratify, but it’s a little awkward since if you specify the path option, it’ll ignore the id and parentId, so I thought maybe it would be better to separate the two. That said I might try a variant of this PR that is more minimal.

[mbostock]

Closing this in favor of #185 to be more conservative, but I do think it may be worth adopting this options-based approach in the future. That said it would be a huge effort and would cause churn in the examples even if we do maintain backwards-compatibility (which I would strongly support doing).