replace dc.utils.toHierarchy with d3.stratify

[gordonwoodhull]

As noted by @kum-deepak in #1388:

Can https://github.com/d3/d3-hierarchy/blob/master/README.md#stratify replace dc.utils.toHierarchy?

Yes. It is probably more robust. It looks like the main differences are that it doesn't provide the path as one of the members, and it uses name instead of key. I'm sure dc.sunburstChart could adapt to these changes.

The more we use d3 utilities instead of reinventing them, the better off we are, for robustness and interpretability of the code.

[ialarmedalien]

I have been looking at this as I've got some other hierarchical graphs that I was thinking of adapting -- the main difference between dc.utils.toHierarchy and stratify is that the former adds in missing parent nodes, whereas stratify throws an error. If dc.js is to use stratify, there would need to be some little utility that filled in those gaps in the hierarchy, and I'm not sure that there is a way to check for missing parents without constructing the tree!

[kum-deepak]

Thanks @ialarmedalien! I can see that root node may be missing in some cases. Are there other cases where parent node might be missing?

[ialarmedalien]

Yes -- I think all the examples (in web/examples) create "fake" parent nodes.

[gordonwoodhull]

Maybe it is more like d3.nest with the number of keys dependent on the longest path in the data set. (And with parent nodes annotated with the current path.)

It doesn't look like dc.utils.toHierarchy will work correctly if the parents are present, unless the parent is before all of its children. It will blindly append a leaf whenever it runs out of path elements. So maybe it would be good to specify that the parents should not be present? I'm not sure.

[kum-deepak]

I will suggest few test cases to cover this function. This would allow us to test out any replacement that we may try.

[ialarmedalien]

I have made a set of changes to the sunburst chart in my fork of dc.js; I created a hierarchyMixin which handles converting flat data to a tree structure using d3.stratify, and rewrote the existing code to use the tree structure generated by d3.stratify. chart.formatData() takes a flat data array and a d3 layout function; it calls chart.stratify(), which adds in missing parent nodes and runs d3.stratify on the data to produce a d3 hierarchy. It then applies the layout function to the tree and returns an array of the nodes in the tree structure. There is also a tiny partitionMixin with partition chart-specific functions as I'm interested in adapting the rectangular partition chart for dc.js.

I also did some work to reduce the large degree of code repetition between the sunburst and pie charts. I have moved the code relating to legend elements into legendableMixin and the rest of the shared code into pieTypeMixin. Some of the mixin functions had to be overridden in dc.pieChart and dc.sunburstChart, and there is a little bit of hackiness to keep some similar-but-not-quite-the-same functions in pieTypeMixin. As a result of this merge, the sunburst chart now supports external labels and label paths (like pie charts), and I fixed a bug that meant that the colours in the legend bore no resemblance to those in the sunburst chart. The test suite also passes.

ETA: I notice that the sunburst chart uses the capMixin, but when I try setting a cap, it throws an error. Does it need to use the capMixin?
ETA 2: have hacked a fix so cap is now usable.