Added density reducer to bin/group/hexbin

[wirhabenzeit]

This is a pull request addressing https://github.com/observablehq/plot/issues/1940, see also the discussion in https://talk.observablehq.com/t/normalised-histogram-in-observable-plot/8576

Basically I added a new reduceDensity reducer which computes the density per series/facet. This is slightly different from the reduceProportion (with or without the facet scope) reducer (when not supplied with a value)

• reduceDensity does normalise by the bin size
• reduceDensity normalises by group rather than globally or by facet.

I forked the notebook https://observablehq.com/@fil/plot-normalized-histograms here https://observablehq.com/d/fb0d876105777d59 to illustrate the functionality. I also added a test plot in test/plots/density-reducer.ts

The main change to existing code is that in src/transforms/group.js, src/transforms/bin.js and src/transforms/hexbin.js need to call the reducer scope on a per group level as in

for (const o of outputs) o.scope("group", I);

I noticed that this new density reducer can also be used as a replacement for proportion-facet in

• test/plots/athletes-sport-weight.ts
• test/plots/hexbin-r.ts

where (at least to me) it also makes sense semantically.

Among the tests this would leave

• test/plots/penguin-species-island-relative.ts

as the only place where proportion-facet is used. Here density does not makes sense on a group level due to the grouping by fill.

One could also consider a more customisable reducer allowing to specify the normalisation scope, but I could not come up with a satisfying syntax. Something like

..., y: {reducer: "density", scope: "group"}, ...

feels a bit too verbose given that most reducers do not have/need any scope.