d3
bisectCenter naming
I find it unintuitive that bisectCenter can return a value that is less than both bisectLeft and bisectRight. Based on how bisectLeft and bisectRight work, I would expect bisectCenter to either match bisectLeft and bisectRight (if the value doesn't exist in the array) or give me the index in the middle of a sequence of equal values. Reading the documentation and #138, I now understand that this method is accomplishing something different.
To me, it would make sense to rename this method to something like closestIndex (even though the implementation uses a bisector). One could also rename bisector to locator or something more general and then have bisectLeft, bisectRight, and closestIndex instead of .left, .right, and .center.
