fix(forecast): resolve time grain robustly for Prophet forecasting

[yousoph]

SUMMARY

Enabling Forecast (Advanced Analytics → Predictive Analytics) on a time-series chart could fail with an opaque 500:

prophet() missing 1 required positional argument: 'time_grain'

Root cause: the prophet post-processing op is dispatched via getattr(pandas_postprocessing, operation)(df, **options) in QueryObject.exec_post_processing. The frontend prophetOperator set time_grain: formData.time_grain_sqla; when that value is undefined — e.g. a saved/dashboard chart whose Time Grain control was cleared to "None", or a chart using the generic x-axis where the grain lives on the adhoc column's timeGrain popover instead of the panel control — JSON.stringify drops the key. The backend then calls prophet() without time_grain and Python raises a raw TypeError at call time, before the graceful if not time_grain: raise InvalidPostProcessingError(_("Time grain missing")) guard inside prophet() can run.

Fix (both ends):

• Frontend (prophetOperator.ts): resolve the effective time grain in priority order — adhoc x-axis column timeGrain → queryObject.extras.time_grain_sqla (picks up dashboard-applied grains) → formData.time_grain_sqla → P1D fallback (matching the time_grain_sqla control default in sharedControls), so forecasting works instead of erroring.
• Backend (prophet.py): make time_grain an optional kwarg (Optional[str] = None), so any remaining path that can't determine a grain surfaces the existing readable "Time grain missing" InvalidPostProcessingError (400) instead of an unhandled TypeError (500). No positional callers exist — the op is always dispatched with keyword options.

BEFORE/AFTER

Before: forecast on a chart without time_grain_sqla in its form data → HTTP 500, raw TypeError message shown in the chart.

After: the grain is resolved from the x-axis column / query extras / form data (falling back to P1D) and the forecast renders; if a grain still can't be determined server-side, the user gets a readable "Time grain missing" error (400).

TESTING INSTRUCTIONS

pytest tests/unit_tests/pandas_postprocessing/test_prophet.py -v
cd superset-frontend && npx jest packages/superset-ui-chart-controls/test/operators/prophetOperator.test.ts

New tests:

• Backend: dispatch-path call with no time_grain and explicit time_grain=None both raise InvalidPostProcessingError ("Time grain missing") instead of TypeError.
• Frontend: grain resolution from adhoc x-axis column, from queryObject.extras, and the P1D fallback when no grain is present anywhere.

To reproduce manually: save a Time-series Line Chart, clear Time Grain to "None", enable Forecast under Advanced Analytics, run the chart.

ADDITIONAL INFORMATION

• Fixes a customer-reported issue; follow-up to fix(pandas-postprocessing): handle prophet errors and validate minimum data points for forecast #41180 which fixed separate errors inside prophet fitting — this failure happened before prophet was ever invoked.
• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration
• Introduces new feature or API
• Removes existing feature or API

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[Copilot]

Pull request overview

This PR fixes a forecasting failure mode where the Prophet post-processing step could be invoked without a time_grain, causing an unhandled backend TypeError (500) instead of a readable validation error. It makes time grain resolution more robust on the frontend and makes the backend Prophet post-processing function resilient to missing time_grain in the dispatched options.

Changes:

• Frontend: resolve the effective time_grain from (1) adhoc x-axis column timeGrain, (2) queryObject.extras.time_grain_sqla, (3) formData.time_grain_sqla, falling back to daily (P1D).
• Backend: make time_grain optional in prophet() so missing/None hits the existing "Time grain missing" InvalidPostProcessingError path rather than raising a raw TypeError.
• Tests: add regression coverage for missing/None time_grain on the backend, and for the new frontend resolution/fallback behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/unit_tests/pandas_postprocessing/test_prophet.py	Adds regression tests ensuring missing/None time_grain yields a readable InvalidPostProcessingError.
superset/utils/pandas_postprocessing/prophet.py	Updates Prophet post-processing signature to accept optional time_grain and preserve graceful error behavior.
superset-frontend/packages/superset-ui-chart-controls/src/operators/prophetOperator.ts	Resolves effective time_grain from x-axis/query extras/form data with a P1D fallback to avoid dropped undefined options.
superset-frontend/packages/superset-ui-chart-controls/test/operators/prophetOperator.test.ts	Adds tests for time grain resolution sources and the daily fallback when no grain is provided.

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[rusackas]

Thanks @yousoph, LGTM.

Curious about the P1D fallback: for a chart where Time Grain was deliberately cleared to None, assuming daily could put the forecasted points at the wrong cadence versus the actual data. Not blocking, just wondering if that's worth a follow-up.

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