This adds d3.layout.pie, which can be used to compute the start and end angles
for arcs given an arbitrary array of data. In addition, d3.svg.arc now has a
`centroid` method that computes the center of the arc, useful for labeling.
Together these changes greatly simplify the donut & pie examples.
This commit also includes a `map` method for selections, which is similar to the
`filter` method. This allows you to map the data bound to the current selection.
I'm not currently using it, but it seems like a useful feature for those cases
where you want to rebind the current selection to different (but related) data.
The data join is now specified as a single function of data, as with all other
properties. This allows the key to be computed on the previously-bound data,
rather than requiring the key to be serialized into the DOM (say, as an
attribute). In the case that there is no previously-bound data, it is still
possible to access the associated node as the `this` context.
The `enter` operator no longer performs an append. For symmetry with the `exit`
operator, you must call `append` after obtaining the entering selection. This
requires a tiny bit more code, but should make the code more clear. Also, it
provides an opportunity to use a different instantiation operator, such as the
new `insert` operator. This takes a second argument, which is a selector for the
insert-before reference element. For example, the selector ":first-child" will
prepend nodes.
The `empty` operator allows you to query whether a selection is empty (i.e.,
contains zero matching nodes).
It's a lot more predictable if we always use selectAll to determine the parent
node for a given group, rather than trying to infer parent nodes per node
during select.
Michael Bostock