Add third-party library 'webtreemap' for use in tools.

third_party/webtreemap/src/README.markdown

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+# webtreemap
+
+A simple treemap implementation using web technologies (DOM nodes, CSS
+styling and transitions) rather than a big canvas/svg/plugin.
+
+Play with a [demo][].
+
+[demo]: http://martine.github.com/webtreemap/demo/demo.html
+
+## Creating your own
+
+1. Create a page with a DOM node (i.e. a `<div>`) that will contain
+   your treemap.
+2. Add the treemap to the node via something like
+
+        appendTreemap(document.getElementById('mynode'), mydata);
+3. Style the treemap using CSS.
+
+### Input format
+
+The input data (`mydata` in the overview snippet) is a tree of nodes,
+likely imported via a separate JSON file.  Each node (including the
+root) should contain data in the following format.
+
+    {
+      name: (HTML that is displayed via .innerHTML on the caption),
+      data: {
+        "$area": (a number, in arbitrary units)
+      },
+      children: (list of child tree nodes)
+    }
+
+(This strange format for data comes from the the [JavaScript InfoVis
+Toolkit][thejit].  I might change it in the future.)
+
+The `$area` of a node should be the sum of the `$area` of all of its
+`children`.
+
+(At runtime, tree nodes will dynamically will gain two extra
+attributes, `parent` and `dom`; this is only worth pointing out so
+that you don't accidentally conflict with them.)
+
+### CSS styling
+
+The treemap is constructed with one `div` per region with a separate
+`div` for the caption.  Each div is styleable via the
+`webtreemap-node` CSS class.  The captions are stylable as
+`webtreemap-caption`.
+
+Each level of the tree also gets a per-level CSS class,
+`webtreemap-level0` through `webtreemap-level4`.  These can be
+adjusted to e.g. made different levels different colors.  To control
+the caption on a per-level basis, use a CSS selector like
+`.webtreemap-level2 > .webtreemap-caption`.
+
+Your best bet is to modify the included `webtreemap.css`, which
+contains comments about required and optional CSS attributes.
+
+## Related projects
+
+* [JavaScript InfoVis Toolkit][thejit]
+
+[thejit]: http://thejit.org/