alex-poor opened a new issue, #40786:
URL: https://github.com/apache/superset/issues/40786
### Bug description
The deck.gl **Polygon** chart legend renders bucket labels with **shared
endpoints**, so consecutive bins appear to overlap. With default settings (no
manual break points), the legend shows e.g.:
```
1 - 81
81 - 212
212 - 369
```
Every interior breakpoint (`81`, `212`, …) appears in **two** adjacent
labels — once as an upper bound, once as a lower bound — which reads as
overlapping ranges and is confusing.
**This is a cosmetic / UX-clarity issue, not a correctness bug.** The
underlying binning and coloring are correct: colors are assigned with d3's
`scaleThreshold`, which treats each breakpoint as a half-open cut point, so a
value of exactly `81` lands in exactly one bucket (`81–212`). No polygon is
double-counted or mis-colored. Only the legend *label text* is ambiguous.
### Root cause
In `superset-frontend/plugins/preset-chart-deckgl/src/utils.ts`,
`getBuckets()` builds each label by pairing adjacent breakpoints:
```js
const range = `${breakPoints[i]} - ${breakPoints[i + 1]}`;
```
This always reuses each interior breakpoint in two labels. The color scale
(`getBreakPointColorScaler`, same file) uses `scaleThreshold`, which is
half-open and unambiguous — so the labels misrepresent the (correct) bucketing
logic.
### Scope
- Affects the deck.gl **Polygon** layer only — it is the only chart in the
frontend that renders a **discrete numeric-range legend** (`scaleThreshold`
appears nowhere else in `superset-frontend`).
- Country Map / World Map avoid this by using a **continuous linear color
scale** with no discrete-range legend.
- ECharts charts use **categorical** legends (names, not ranges), so they
are unaffected.
- Independent of the `Number of buckets` vs `Bucket break points` controls —
setting explicit break points changes *where* the bins are, but labels still
share endpoints.
### Expected behavior
Legend bins should read as a clean, non-overlapping partition. There is no
existing internal convention to match (Polygon is the only discrete-range
legend), so candidate formats:
1. **Edge-operator notation** (common GIS/choropleth convention — ArcGIS,
Datawrapper): `< 81`, `81 – 212`, `212 – 369`, `> 369`
2. **Interval notation:** `[1, 81)`, `[81, 212)`, `[212, 369]`
3. **Render the boundary number once between touching swatches**
(gradient-bar style) rather than as isolated `a - b` rows.
> Note: simply incrementing the lower bound (`82 - 212`) is *not* correct in
general — breakpoints and metric values can be fractional, so a value like
`81.5` would fall into a labeling gap.
### How to reproduce
1. Create a deck.gl **Polygon** chart with a numeric metric.
2. Leave **Bucket break points** empty (use default **Number of buckets**).
3. Observe the legend — adjacent bins share endpoints (`1 - 81`, `81 - 212`,
…).
### Superset version
master / latest (also observed on 5.0+).
### Related (different) issues
- #34747 — legend *limits* miscalculated (rounding the max bound); a
separate, already-fixed bug.
- #7264, #35142 — other deck.gl legend fixes; none address label formatting.
No existing open issue/PR covers the shared-endpoint label formatting.
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