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 threshold scale invertExtent. Related #1201.
Merge branch 'quantize-invert' of github.com:scottcheng/d3 into invert-extent