Related Issues

• Added [feat]: Add --stack for 2D grouped bar/line charts #185

Summary

• Added opt-in --stack support for 2D grouped bar and line charts.
• bar --stack renders stacked bars.
• line --stack renders stacked area lines.
• Default behavior remains grouped side-by-side / normal lines.
• Stack is skipped for z data and log scale.

Changes

• Added StackFlag in Go chart flags.

• Added stack JSON config to BarConfig and LineConfig.

• Registered --stack on bar and line commands.

• Added rule checks:

  • requires x,y
  • excludes z
  • skips when scale=log

• Added stack?: boolean to UI chart config types.

• Threaded stack through active chart shape, chart options, and ChartCard.

• Updated bar renderer to emit stack: 'total'.

• Updated line renderer to emit stack: 'total' and areaStyle: {}.

• Added StackControl.vue and Settings panel toggle.

• Updated settings registry and tests for stack visibility.

• Updated docs with stacked bar and stacked area examples.

• Regenerated pkg/template/vizb-ui.gen.go.

• Resolved merge conflicts with upstream horizontal bar and smooth line
  features preserved.

Test Result

Ran validations below

task build:ui
task build:cli

cd ui
pnpm test
pnpm typecheck
pnpm format:check

Case 1 (Bar)

./bin/vizb bar examples/csv/sales.csv \
    -g region,category \
    -p x,y \
    --stack \
    -o /tmp/vizb-stacked-bar.html

Case 2 (Line)

./bin/vizb line examples/csv/sales.csv \
    -g region,category \
    -p x,y \
    --stack \
    -o /tmp/vizb-stacked-line.html

Case 3 (log scale + stack, should warning and ignore stack)

./bin/vizb bar examples/csv/sales.csv -g region,category -p x,y --stack
  --scale log -o /tmp/log.html