Added Variance and Corrected Standard Deviation to core array functions.
renamed sd to deviation #1073
added exit criteria for variance that returns NaN on an empty array
refactored results with NaN array and empty arrays to fit the results of R
using an evaulation array for accessor functions and ignoring NaN, undefined and null
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.