The set of chart kinds is open, and a kind is added when it answers a question no kind there answers better

Status: accepted, supersedes ADR-0032 in two points Date: 2026-10

ADR-0032 called the set of chart kinds closed, with "there is no renderer interface for third parties" as its sign. The need for a ranking with long names (11 October 2026) showed that the sentence shuts more than its reasons do: the reasons are against six particular kinds, not against a seventh.

The set of kinds is open. A kind is added when it answers a question that no kind there answers better, and each gets an ADR that names the question. The library builds the kinds; there is no renderer interface for third parties.

The test

A kind's ADR:

1. states the question in the reader's words;
2. names the nearest kind and what answering with it costs the caller;
3. shows that nothing is drawn that was not measured;
4. says how keyboard and screen reader reach every value;
5. says how it sits beside ADR-0001: on affine axes or, with none (Sankey, treemap), on what.

This ADR decides no kind. It decides what the deciding must show.

The exclusions that stand

Their reasons are unchanged: pie and donut (a share reads better from sorted bars, pareto()), radar (a profile reads better as a line per axis), candlestick (candles are for markets), violin (a density estimate draws values nobody measured), a log scale (every scale is affine, ADR-0001).

The kinds, in order, and what each must answer

• Horizontal bars, first. A ranking with long names, read by name. Today a Matrix or a Table. Must show: names on y without a rotated label (the category axis of ADR-0052 on y), stacking and data table unchanged. This supersedes ADR-0032's exclusion ("bars grow along the y axis"): a kind named as the first to come cannot also stand under "never".
• Sankey, a case of its own. What flows where: material from stage to stage, rejects by cause. Today a stacked bar per stage, which loses the links. Must show: conservation (in equals out, or the gap named as a finding), a table of links as the alternative, a walk over nodes then links.
• A bullet graph, in core beside Meter. One value against a target and bands. Today Meter plus a limit. Must show what it says that Meter does not (a target mark, a comparison) and that it has a verdict, as Meter has and ProgressBar has not. A component, not a chart kind.
• A treemap, later. The parts of a whole in two levels. Today grouping with aggregates in the table. Must show why area beats a sorted bar.

What it supersedes

• ADR-0032 stays in its other points. Its sentence "the set of kinds is closed" falls, and so does the exclusion of horizontal bars.

Considered Options

• Stay closed and decline horizontal bars again. The maintainer is the caller, which is ADR-0032's own door.
• Open it for horizontal bars only. The next request reopens it.
• A renderer interface for callers. Each kind would then need a stable contract for scene, hit model, accessibility and data table, for kinds the library would not test.

Consequences

• Each kind costs an ADR, a walk and a data table - the price of the test.
• A request for pie is still answered with ADR-0032 and this one.

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