adding step-middle as interpolation method
change step-middle to just step
cleaner formatting for step interpolation method
Add brush.clamp method to specify to clamp extent to range or not
Make clamp implementation more similar to extent
Don't consider dragging when determining to clamp or not
Add scale.quantize.invert
Rename quantize.invert to quantize.invertExtent
Use d3_Map instead of scale.ordinal in scale.quantize.invertExtent
Add test for scale.quantize.invertExtent
Implement scale.quantize.invertExtent with Array.indexOf
Add test for scale.quantize.invertExtent: arbitrary range values
scale.quantize.invertExtent returns null when y is not in the range
Add test for scale.quantize.invertExtent: out-of-range values and duplicate values in the range
Add quadtree.extent. Fixes #1207.
Add {cluster,tree}.nodeSize. Fixes #317.
Allow call() to be called on selections following enter()
Add contentParserCallback to d3.xhr constructor

Fixes race condition that exists where the xhr.onload fires before
xhr.response has had a change to be set. This causes d3_json for example
to not get called.
Update to Mike's approach
fix dumb merge issues