Making per-series chart styling reachable

[rlei-odes]

I'm new to contributing here. Using Superset with a real dataset for the first
time, I ran into a wall trying to style individual series, and I think the wall
is smaller than it looks. Two small PRs to offer, and one question I'd like
opinions on before writing anything.

What I'm trying to build

Grey for previous years, dark for actual, hatched for forecast, outline for
plan. A common convention in business reporting, recently formalised as ISO
24896. The point is that the fill carries the meaning, so the chart reads
without the legend and colour stays free for something else.

The screenshot is from the plugin I made, see links below. My goal would be to get closer to achieving this by adding the necessary features.

Where it stands

ECharts does all of this natively, and Superset ships it. The two properties I
need are itemStyle.borderColor + borderWidth for the outline, and
itemStyle.decal for the hatching.

Neither has a control, so the only way to set them is Customize → ECharts
Options (JS object literals), the free-text box where you can hand-write ECharts
options that get merged over the ones Superset computed. What you write there is
checked against an allowlist schema first, and anything not on it is dropped.

• Outline is on that allowlist already.
• Hatching isn't - decal is the only member of itemStyle missing from
  itemStyleSchema, which looks accidental.

But neither is actually reachable, for a reason that has nothing to do with the
allowlist: both are per-series properties, so setting one means writing a
series key. mergeCustomEChartOptions replaces arrays instead of merging
them:

// mergeCustomEChartOptions.ts
// Replace arrays and primitive values directly
(result as PlainObject)[key] = customValue;

So supplying series discards the series Superset just computed, data
included. The replacement can't carry its own data either, since data isn't in
seriesSchema (deliberately, and rightly).

So that box works well for top-level objects like xAxis and yAxis, which do
merge, and cannot express anything per series at all.

Two PRs I'd like to offer

Both small, additive, and useful on their own:

1. Allow decal in the options schema. One property on itemStyleSchema,
   mirroring ECharts' DecalObject. borderColor and lineStyle.type are
   already allowed and a decal is the same kind of value, so this mostly closes
   an inconsistency. On its own it changes nothing that renders, see the
   question below.
2. Chart chrome as controls - gridlines, axis ticks, axis labels, axis line
   colour and width. Currently hardcoded as splitLine: { show: !isSmallChart }
   in the transform. Useful well beyond my use case; the sparse look in the
   chart above isn't reachable today without hand-written JSON.

The question

Neither of those makes per-series styling work. That needs the array problem
solved, and I can see two ways:

a) Merge series element-wise instead of replacing it. Then
{ series: [{ itemStyle: { decal: {...} } }] } would decorate the first series
without destroying its data, and both outline and hatching become usable
immediately through a control that already exists. This looks small, and ECharts
itself merges by index or id in setOption. The obvious risk is that it changes
behaviour for anyone currently relying on replacement, and series indices are
positional, so it may be fragile when the series count changes.

b) A real control for per-series styling, rather than routing it through
JSON. More work, better UX, and it raises a design question I don't want to
answer alone: how should a series be addressed? Matching on the rendered series
name is the obvious answer and I'd argue against it. Names are composed from
the metric and groupby values, so a rule silently stops matching the moment
someone adds a dimension.

Is either direction something Superset wants? I'm happy to do the work, but I'd
rather find out now than arrive with a large PR. And if the answer is "this
belongs in a plugin", that's useful to know too.

Working code

All of the above is running in a fork as a self-contained plugin:

• Diff against master
• What's added versus merely surfaced

[rlei-odes]

Two of these are now open:

• feat(plugin-chart-echarts): allow decal patterns in custom ECharts options #43427 — allow decal in the ECharts options schema. Closes the one-property gap; changes nothing that renders on its own.
• feat(plugin-chart-echarts): add gridline and axis tick controls #43428 — Gridlines and Axis ticks controls, mirroring the Minor Split Line and Minor ticks controls that already exist on the same charts. Both default to on, so existing charts are unchanged.