Support side-by-side grouped bar by disabling stacking when color dimension exists

[su-jin02]

Problem

In Graphic Walker, when creating a bar chart with:

• x: categorical field (e.g. Year)
• y: quantitative field (e.g. Revenue)
• color: categorical field (e.g. Company)

the current behavior always results in a stacked bar (or normalized / centered stack),
even when the analytical intent is to compare values side-by-side within the same x category.

There is currently no way to render a true "grouped (side-by-side) bar" where
each color category produces an independent bar within the same x value.

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Observation

From inspecting the generated Vega-Lite spec and renderer behavior,
the key difference between stacked vs grouped bar appears to be whether
the y encoding has stack enabled.

In Vega-Lite, a grouped bar can be achieved by explicitly disabling stacking:

y: {
  field: "...",
  type: "quantitative",
  stack: null
}

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Minimal logic proposal

A minimal and non-invasive way to support grouped bars could be:

1. Detect grouped-bar intent

const isGrouped = !!(colorField && xField && colorField !== xField);

2. Disable stacking on y encoding for grouped bars

if (isGrouped && mark === "bar") {
  yEncoding.stack = null;
}

This does not introduce facet, multi-view, or additional UI complexity.
It only changes the stacking behavior when a color dimension exists
and a grouped comparison is desired.

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Why this works well for Graphic Walker

• Uses standard Vega-Lite semantics (no custom renderer required)
• Minimal change in spec generation
• No need to expose raw "facet" concepts in the UI
• Aligns with common BI expectations (Tableau-style grouped bar)
• Keeps existing Stack / Normalize / Center modes intact

A new UI option such as "Grouped (side-by-side)" could internally map to
this behavior without changing the overall UX philosophy.

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Expected result

For:

• x = Year
• y = Revenue
• color = Company

Bars should render side-by-side within each Year,
instead of being merged into a single stacked bar.

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If this direction makes sense, I’m happy to help with a PoC or PR.
Note: This proposal focuses on rendering behavior only.
UI exposure (toggle or preset) can be discussed separately.