facetAnchor function?

If facetAnchor was a function of the facet’s values and index {fx, fy, fi}, it would be simpler to, for example, make a "zebra frame" that highlights every second facet.

Currently it's possible to do so with a render transform which checks index.fi to decide whether to call next(…), e.g.:

Plot.frame({
  render: (index, scales, values, dimensions, context, next) => 
    index.fi % 2 ? next(index, scales, values, dimensions, context) : null,
  fill: '#eee'
})

(feature suggested by @mayagans)

The issue names no files or tests. Start by tracing the facet rendering and render-transform entry points that expose index.fi, then examine how a facetAnchor function could receive {fx, fy, fi}; done means the zebra-frame example can use the proposed API without its custom transform.