Includes, as the first behavior, a pan & zoom behavior. The canvas can be panned
by dragging the mouse, and zoomed using the mousewheel (or by double-click). By
listening to redraw events, users can decide whether to implement geometric
zooming (such as by setting the "transform" attribute on an `svg:g` element) or
semantic zooming (by changing the domain of a scale object and repositioning
elements).
This commit also includes two bug fixes. The `d3.format` class now properly
groups thousands of negative numbers, and supports the sign specifier. The
unicode minus symbol \u2212 is used for negative values. The `d3.scale.pow`
class now properly handles negative numbers, as well.
Michael Bostock