This fixes a bug in the pie layout sorting where we want the sort order to
affect the layout, but not the order in which arcs are rendered—and furthermore
we want the order of arcs to always match the order of data. (If you want to
sort the data, do that before it is passed to the layout.)
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.