fix: bar and area chart stacked toggle not working

[Ju4n5e]

Summary

• Fixes the stacked toggle having no effect on bar and area charts
• The UI toggle sets config.stacked, but BarChartTransformer and AreaChartTransformer only checked config.mark?.stack (which is never set by the config parser)
• Added || config.stacked === true to both transformers so the toggle takes effect

Fixes #556

Details

The config parser (vizConfigFromProps) maps the stacked UI property to config.stacked (top-level). However, both BarChartTransformer.transform() (line 140) and AreaChartTransformer.transform() (line 98) only check config.mark?.stack, which is never populated.

The legacy render path in D3VisualizationEngine correctly checks the top-level config.stacked, but the newer renderWithTransformedData path (which takes priority when transformedData exists) relies on the transformers, which miss it.

Test plan

• Create a bar chart with a color field (e.g., grouping by Priority)
• Toggle Stacked to On — bars should stack vertically instead of side-by-side
• Toggle Stacked to Off — bars should return to grouped (side-by-side)
• Repeat with an area chart to verify the same fix

🤖 Generated with Claude Code