Conditional (threshold) formatting for the Time-series Bar chart

[gkneighb]

Summary

Extend Superset's existing conditional formatting to the ECharts Time-series Bar chart (echarts_timeseries_bar): let users color individual bars by threshold rules, the same way the Table chart already does.

Motivation

In migration/feedback sessions, threshold-driven bar coloring (e.g. "bars below target in red, at/above in green") is a recurring ask — it's a standard Tableau capability. Superset already ships this pattern:

• Table chart: ConditionalFormattingControl (conditional_formatting control; >, <, ≥, ≤, between + color choices).
• Big Number: conditional formatting on the value.

The Time-series Bar chart — one of the most-used viz types in migrated dashboards — has no equivalent, so users fall back to manual workarounds or leave it in Tableau.

Proposed scope (v1)

• One chart type: echarts_timeseries_bar only. Line/area/scatter are out of scope for v1 (they're separate viz types with their own control panels, so this is a clean, isolated addition).
• Reuse the existing control (ConditionalFormattingControl / ConditionalFormattingConfig) — same operators and color options. No new control type.

Proposed behavior

• The rule's target is a metric on the chart (the Table targets a column; a chart has none, so the target dropdown is fed the chart's metrics via mapStateToProps). This is the one adaptation of the shared control.
• At render, each data point whose metric value satisfies a rule gets that rule's color via ECharts itemStyle; non-matching points keep the series color.
• Multiple rules: last matching rule wins (matching Table semantics).

Open questions for the community

1. Metric-as-target adaptation of ConditionalFormattingControl — any objection, or a preferred pattern already used elsewhere?
2. Stacked bars: should v1 support per-segment coloring, or explicitly scope stacked out (per-segment threshold coloring can be visually ambiguous on a stacked total)?
3. Scope: is bar-only the right v1, or would maintainers prefer this land across all Time-series series types (bar/line/area) at once?
4. Sizing: does this read as an incremental PR (extending an existing control to one chart type), or does anyone see it as SIP-warranting? Confirming before opening a PR.

Happy to implement — opening this first to confirm direction and sizing.