more treemap

[Fil]

Several elements that might help with treemaps.

clipping to a rect (#1109)

There are two issues here:

• how do we implement the rect clipping? Since Chromium doesn't support the view-box keyword we have two possibilities: either adding a svg element (which I did here), or a clipPath with a unique id and a url().
• Is this not an indication that we want a "boxtext" mark, which would combine rect+text in a smarter way (in particular, allowing to anchor the text to either side of the rect)

node:branch and node:size helpers

node:branch retrieves the id of the top-level branch and color the rects; the original example was using the internal d3 hierarchy object, which shows a leaky abstraction; should this leak be closed?

node:size shows the value.

a default text and title

default text is the node:name, default title is node:name + node:size

I'm happy that it generally works, but it opens a few questions about how to make this as generic as possible.

faceting

treemap areas are comparable across facets

todo:

• a normalize option, or “proportion-facet” reducer?