fix(chart): re-enable cross-filtering on categorical X-axis when groupby is empty (backport to 6.0)

[jtjenkins]

Summary

Cross-filtering is silently broken in 6.0.0 for the most common bar chart pattern: charts that use a categorical x_axis column with no groupby series breakdown.

This is a backport of the fix already merged to master (issue #25334) to the 6.0-bug-fixes branch.

Root cause

EchartsTimeseries.tsx only emits a crossFilter object when hasDimensions is true (groupby is non-empty):

crossFilter: hasDimensions
  ? getCrossFilterDataMask(seriesName)
  : undefined,   // ← always undefined when groupby is []

This was fine before PR #31582, which moved the categorical X-axis column from groupby to the dedicated x_axis field. After that migration, charts that only define an X-axis category have groupby = [], so hasDimensions = false and cross-filtering is permanently disabled — both right-click "Add Cross Filter" (appears grayed out) and left-click selection.

Fix

Introduces canCrossFilterByXAxis = !hasDimensions && xAxis.type === AxisType.Category and a parallel getXAxisCrossFilterDataMask helper that builds the filter on xAxis.label from the clicked bar's category value (props.data[0]). Both the click and contextmenu handlers are updated to use this path when groupby is empty but the X-axis is categorical.

Toggle/deselect uses Object.values(selectedValues), consistent with the existing getCrossFilterDataMask behaviour.

Changes

• superset-frontend/plugins/plugin-chart-echarts/src/Timeseries/EchartsTimeseries.tsx

Testing

Verified end-to-end in a Superset 6.0 instance:

• Bar chart with x_axis = clinical_stage, groupby = []
• Right-click → "Add Cross Filter" is now enabled (was grayed out)
• Left-click on a bar applies clinical_stage IN [value] and all connected charts update
• Clicking the same bar again deselects (toggle behaviour)
• Charts with groupby set continue to use the original dimension-based cross-filter path

Notes

The equivalent fix was already applied to master (see issue #25334). This PR backports only the cross-filter change to 6.0-bug-fixes, with no unrelated changes from master.

Fixes #25334 (backport to 6.0)

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