Could Plot.auto make the Simpsons cell heatmap?

[tophtucker]

Selecting a bar override on "season" vs "number_in_season" makes a bunch of zero-width, zero-height rects (and a working tooltip with some extraneous intervals): https://observablehq.com/d/58a592d7a3c97fc8

Plot.auto(simpsons, {x: "season", y: "number_in_season", color: "imdb_rating", mark: "bar"}).plot()

I wish it made this:

Plot.cell(simpsons, {x: "season", y: "number_in_season", fill: "imdb_rating"}).plot()

If there's no reducer / no binning by which to give the rects extent, could we infer that we could treat x and y as discrete and use cell instead? Though maybe that'd be brittle; it'd kinda depend on the numbers being evenly spaced.

[Fil]

would it be mark: "cell", or "interval: 1" (which it should then do in both dimensions)

[mbostock]

I think this is a straight-up bug, not the intended behavior. I believe the code assumes that if we’re using the rect mark, that there’s a color reducer (and hence that x and y are binned). But that isn’t the case in the provided snippet.

Here are some options that I don’t immediately reject:

1. Choose cell instead of rect as the mark implementation.
2. Ignore the mark option and use dot instead.
3. Throw an error explaining that a reducer is required. (I don’t like this one.)
4. Add an implicit color reducer (median? mode? first?) to enable binning.
5. Add an implicit interval option? (But how would we know what interval to use…)

To me (1) choosing cell instead of mark is likely the easiest fix and seems like the place to start. I think it’s this line here, but you have to check whether a colorReduce was specified.

plot/src/marks/auto.js

Line 134 in 16c75f4

: rect;

(I think we’d still chose cell if a sizeReduce was specified? But the inputs don’t make much sense since the bar mark doesn’t support a size encoding.)

I would not introduce a cell mark option as @Fil suggested. A big value-add of the auto mark is that the differences between the rect marks are ignorable and in the chart cell we wouldn’t want the user to choose between cell and bar (and rect).