alternative to bin{filter} : bin {transform}

[Fil]

what if we did something totally different and passed the actual bin to a transform option?

• the default might be transform: bin => bin.length ? bin : null which would remove empty bins.
• using transform: bin => bin or transform: "identity" ("keep-empty"), would keep empty bins
• using transform: bin => bin.length < 20 ? null : bin would do for athletesHeightWeightBinStroke

Ref #495 (comment)

[mbostock]

Thanks for the suggestion! Closing in favor of #495 (comment).