fix: apply bandPosition to offset channel for grouped bar (so band size works with grouped bars)

examples/compiled/bar_grouped_discrete_bandsize.vg.json

@@ -0,0 +1,142 @@
+{
+  "$schema": "https://vega.github.io/schema/vega/v5.json",
+  "background": "white",
+  "padding": 5,
+  "height": 200,
+  "style": "cell",
+  "data": [
+    {
+      "name": "source_0",
+      "values": [
+        {"category": "A", "group": "x", "value": 0.1},
+        {"category": "A", "group": "y", "value": 0.6},
+        {"category": "A", "group": "z", "value": 0.9},
+        {"category": "B", "group": "x", "value": 0.7},
+        {"category": "B", "group": "y", "value": 0.2},
+        {"category": "B", "group": "z", "value": 1.1},
+        {"category": "C", "group": "x", "value": 0.6},
+        {"category": "C", "group": "y", "value": 0.1},
+        {"category": "C", "group": "z", "value": 0.2}
+      ]
+    },
+    {
+      "name": "data_0",
+      "source": "source_0",
+      "transform": [
+        {
+          "type": "stack",
+          "groupby": ["category", "group"],
+          "field": "value",
+          "sort": {"field": [], "order": []},
+          "as": ["value_start", "value_end"],
+          "offset": "zero"
+        },
+        {
+          "type": "filter",
+          "expr": "isValid(datum[\"value\"]) && isFinite(+datum[\"value\"])"
+        }
+      ]
+    }
+  ],
+  "signals": [
+    {
+      "name": "x_step",
+      "update": "20 * bandspace(domain('xOffset').length, 0, 0) / (1-0.2)"
+    },
+    {
+      "name": "width",
+      "update": "bandspace(domain('x').length, 0.2, 0.2) * x_step"
+    }
+  ],
+  "marks": [
+    {
+      "name": "marks",
+      "type": "rect",
+      "style": ["bar"],
+      "from": {"data": "data_0"},
+      "encode": {
+        "update": {
+          "fill": {"scale": "color", "field": "group"},
+          "ariaRoleDescription": {"value": "bar"},
+          "description": {
+            "signal": "\"category: \" + (isValid(datum[\"category\"]) ? datum[\"category\"] : \"\"+datum[\"category\"]) + \"; value: \" + (format(datum[\"value\"], \"\")) + \"; group: \" + (isValid(datum[\"group\"]) ? datum[\"group\"] : \"\"+datum[\"group\"])"
+          },
+          "x": {
+            "scale": "x",
+            "field": "category",
+            "offset": {"scale": "xOffset", "field": "group", "band": 0.25}
+          },
+          "width": {"signal": "max(0.25, 0.5 * bandwidth('xOffset'))"},
+          "y": {"scale": "y", "field": "value_end"},
+          "y2": {"scale": "y", "field": "value_start"}
+        }
+      }
+    }
+  ],
+  "scales": [
+    {
+      "name": "x",
+      "type": "band",
+      "domain": {"data": "data_0", "field": "category", "sort": true},
+      "range": {"step": {"signal": "x_step"}},
+      "paddingInner": 0.2,
+      "paddingOuter": 0.2
+    },
+    {
+      "name": "y",
+      "type": "linear",
+      "domain": {"data": "data_0", "fields": ["value_start", "value_end"]},
+      "range": [{"signal": "height"}, 0],
+      "nice": true,
+      "zero": true
+    },
+    {
+      "name": "xOffset",
+      "type": "band",
+      "domain": {"data": "data_0", "field": "group", "sort": true},
+      "range": {"step": 20}
+    },
+    {
+      "name": "color",
+      "type": "ordinal",
+      "domain": {"data": "data_0", "field": "group", "sort": true},
+      "range": "category"
+    }
+  ],
+  "axes": [
+    {
+      "scale": "y",
+      "orient": "left",
+      "gridScale": "x",
+      "grid": true,
+      "tickCount": {"signal": "ceil(height/40)"},
+      "domain": false,
+      "labels": false,
+      "aria": false,
+      "maxExtent": 0,
+      "minExtent": 0,
+      "ticks": false,
+      "zindex": 0
+    },
+    {
+      "scale": "x",
+      "orient": "bottom",
+      "grid": false,
+      "title": "category",
+      "labelAlign": "right",
+      "labelAngle": 270,
+      "labelBaseline": "middle",
+      "zindex": 0
+    },
+    {
+      "scale": "y",
+      "orient": "left",
+      "grid": false,
+      "title": "value",
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(height/40)"},
+      "zindex": 0
+    }
+  ],
+  "legends": [{"fill": "color", "symbolType": "square", "title": "group"}]
+}

examples/specs/bar_grouped_discrete_bandsize.vl.json

@@ -0,0 +1,26 @@
+{
+  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
+  "data": {
+    "values": [
+      {"category":"A", "group": "x", "value":0.1},
+      {"category":"A", "group": "y", "value":0.6},
+      {"category":"A", "group": "z", "value":0.9},
+      {"category":"B", "group": "x", "value":0.7},
+      {"category":"B", "group": "y", "value":0.2},
+      {"category":"B", "group": "z", "value":1.1},
+      {"category":"C", "group": "x", "value":0.6},
+      {"category":"C", "group": "y", "value":0.1},
+      {"category":"C", "group": "z", "value":0.2}
+    ]
+  },
+  "mark": "bar",
+  "encoding": {
+    "x": {"field": "category"},
+    "y": {"field": "value", "type": "quantitative"},
+    "xOffset": {"field": "group"},
+    "color": {"field": "group"}
+  },
+  "config": {
+    "bar": {"discreteBandSize": {"band": 0.5}}
+  }
+}

src/compile/mark/encode/offset.ts

@@ -26,7 +26,7 @@ export function positionOffset({
   markDef: MarkDef<Mark, SignalRef>;
   encoding?: Encoding<string>;
   model?: UnitModel;
-  bandPosition?: number;
+  bandPosition?: number | SignalRef;
 }): Offset {
   const channel = `${baseChannel}Offset` as
     | 'xOffset'

src/compile/mark/encode/position-rect.ts

@@ -203,7 +203,16 @@ function positionAndSize(
 
   const vgChannel = vgAlignedPositionChannel(channel, markDef, config, defaultBandAlign);
   const center = vgChannel === 'xc' || vgChannel === 'yc';
-  const {offset, offsetType} = positionOffset({channel, markDef, encoding, model, bandPosition: center ? 0.5 : 0});
+
+  const bandPosition = center
+    ? 0.5
+    : isSignalRef(bandSize)
+    ? {signal: `(1-${bandSize})/2`}
+    : isRelativeBandSize(bandSize)
+    ? (1 - bandSize.band) / 2
+    : 0;
+
+  const {offset, offsetType} = positionOffset({channel, markDef, encoding, model, bandPosition});
 
   const posRef = ref.midPointRefWithPositionInvalidTest({
     channel,
@@ -215,15 +224,7 @@ function positionAndSize(
     stack,
     offset,
     defaultRef: pointPositionDefaultRef({model, defaultPos: 'mid', channel, scaleName, scale}),
-    bandPosition: center
-      ? offsetType === 'encoding'
-        ? 0
-        : 0.5
-      : isSignalRef(bandSize)
-      ? {signal: `(1-${bandSize})/2`}
-      : isRelativeBandSize(bandSize)
-      ? (1 - bandSize.band) / 2
-      : 0
+    bandPosition: offsetType === 'encoding' ? 0 : bandPosition
   });
 
   if (vgSizeChannel) {

test/compile/mark/bar.test.ts

@@ -43,6 +43,45 @@ describe('Mark: Bar', () => {
     expect(props.height).toBeUndefined();
   });
 
+  it('draws vertical grouped bar, with relative band size from config correctly applied for x/width', () => {
+    const model = parseUnitModelWithScaleAndLayoutSize({
+      data: {url: 'data/cars.json'},
+      mark: 'bar',
+      encoding: {
+        x: {field: 'Origin', type: 'nominal'},
+        xOffset: {field: 'SubOrigin', type: 'nominal'},
+        y: {type: 'quantitative', field: 'Acceleration', aggregate: 'mean'}
+      },
+      config: {
+        bar: {discreteBandSize: {band: 0.5}}
+      }
+    });
+    const props = bar.encodeEntry(model);
+    expect(props.x).toEqual({scale: 'x', field: 'Origin', offset: {scale: 'xOffset', field: 'SubOrigin', band: 0.25}});
+    expect(props.width).toEqual({signal: `max(0.25, 0.5 * bandwidth('xOffset'))`});
+    expect(props.y).toEqual({scale: 'y', field: 'mean_Acceleration'});
+    expect(props.y2).toEqual({scale: 'y', value: 0});
+    expect(props.height).toBeUndefined();
+  });
+
+  it('draws vertical grouped bar, with relative band size correctly applied for x/width', () => {
+    const model = parseUnitModelWithScaleAndLayoutSize({
+      data: {url: 'data/cars.json'},
+      mark: {type: 'bar', width: {band: 0.5}},
+      encoding: {
+        x: {field: 'Origin', type: 'nominal'},
+        xOffset: {field: 'SubOrigin', type: 'nominal'},
+        y: {type: 'quantitative', field: 'Acceleration', aggregate: 'mean'}
+      }
+    });
+    const props = bar.encodeEntry(model);
+    expect(props.x).toEqual({scale: 'x', field: 'Origin', offset: {scale: 'xOffset', field: 'SubOrigin', band: 0.25}});
+    expect(props.width).toEqual({signal: `max(0.25, 0.5 * bandwidth('xOffset'))`});
+    expect(props.y).toEqual({scale: 'y', field: 'mean_Acceleration'});
+    expect(props.y2).toEqual({scale: 'y', value: 0});
+    expect(props.height).toBeUndefined();
+  });
+
   it('should draw horizontal bar, with y from zero to field value and bar with quantitative x, x2, and y', () => {
     const x: PositionFieldDef<string> = {field: 'q_start', type: 'quantitative'};
     const x2: SecondaryFieldDef<string> = {field: 'q_end'};