gkneighb opened a new pull request, #42070:
URL: https://github.com/apache/superset/pull/42070
### SUMMARY
Adds a **waterfall** chart type plugin to the MCP `generate_chart` registry
(#39922), the third statistical/analytical addition in this series
(delete/restore lifecycle: #41472 merged / #41842; histogram + box plot:
#41860).
Design notes — incorporating the review outcomes from #41860 up front:
- **form_data verified against the frontend plugin source**, not assumed:
`x_axis` (single step-per-value column, temporal-friendly), single-select
`groupby` breakdown (the Waterfall controlPanel sets `multi: false` — the
config field is a single `breakdown: ColumnRef` accordingly, not a list),
single `metric`, and the display options (`show_total` default `true` per the
controlPanel, increase/decrease/total labels matching `constants.ts`).
`row_limit` defaults to the shared 10000.
- **Native vocabulary accepted**: `groupby` is a validation alias for
`breakdown`, per the codebase's translate-don't-refuse convention.
- **Dimension fields reject metric-style refs**
(`saved_metric`/`sql_expression` on `x_axis`/`breakdown`), consistent with the
tightened validation in #41860.
- **Preview classification included from the start**: `waterfall` maps into
the bar-mark preview bucket (it is a bar construction), avoiding the
misclassification gap found post-hoc on #41860.
- Stale chart-type-count references were swept: the two remaining "7 chart
types" comments in validator/utils docstrings were made count-agnostic so they
can't rot again.
- `get_chart_type_schema` gains the type with two examples; `generate_chart`
docstring and server instructions updated.
### BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF
N/A (MCP service; no UI changes)
### TESTING INSTRUCTIONS
```bash
pytest tests/unit_tests/mcp_service/chart/test_waterfall_chart.py # 13 new
tests
pytest tests/unit_tests/mcp_service/chart/ # full
chart suite
```
Manual: via an MCP client, `generate_chart` with
`{"dataset_id": <id>, "config": {"chart_type": "waterfall", "x_axis":
{"name": "<period_col>"}, "metric": {"name": "<value_col>", "aggregate":
"SUM"}}}`
returns a working preview; add `"breakdown": {"name": "<category_col>"}` for
per-category steps.
### ADDITIONAL INFORMATION
- [ ] Has associated issue:
- [ ] Required feature flags:
- [ ] Changes UI
- [ ] Includes DB Migration (follow approval process in
[SIP-59](https://github.com/apache/superset/issues/13351))
- [ ] Migration is atomic, supports rollback & is backwards-compatible
- [ ] Confirm DB migration upgrade and downgrade tested
- [ ] Runtime estimates and downtime expectations provided
- [x] Introduces new feature or API
- [ ] Removes existing feature or API
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