selectFilter

[Fil]

selectFilter((I, V) => index, {key, ...options})

see discussion at #190 (comment)

=> playground https://observablehq.com/d/c908dd4cadbd9020

related: #138; but it's much more than a filter in the sense that it gets the whole series and can apply, say, ckmeans to do its job.

[Fil]

This could help fix an issue with selectLast etc, which is that they send the last datum of the series even if it's invalid.

using selectFilter((I, V) => I.filter(i => V[i]).slice(-1), {key, ...options}) would give the last valid datum for key

[mbostock]

I’d like to try handling this in #193 by changing maybeTransform to check for a filter option (if there is no transform option), and to promote the given filter function of data up to a transform. There could be similar checks for map and sort convenience shorthands, and we’d need to check for this in the Mark constructor, too.