fix: make timeUnit band respect x2Offset (#8956)

Co-authored-by: GitHub Actions Bot <vega-actions-bot@users.noreply.github.com>

examples/compiled/line_month_center_band_offset.vg.json

@@ -0,0 +1,130 @@
+{
+  "$schema": "https://vega.github.io/schema/vega/v5.json",
+  "background": "white",
+  "padding": 5,
+  "width": 200,
+  "height": 200,
+  "style": "cell",
+  "data": [
+    {
+      "name": "source_0",
+      "url": "data/seattle-weather.csv",
+      "format": {"type": "csv", "parse": {"date": "date"}},
+      "transform": [
+        {
+          "field": "date",
+          "type": "timeunit",
+          "units": ["month"],
+          "as": ["month_date", "month_date_end"]
+        },
+        {
+          "type": "aggregate",
+          "groupby": ["month_date", "month_date_end"],
+          "ops": ["mean"],
+          "fields": ["precipitation"],
+          "as": ["mean_precipitation"]
+        },
+        {
+          "type": "filter",
+          "expr": "(isDate(datum[\"month_date\"]) || (isValid(datum[\"month_date\"]) && isFinite(+datum[\"month_date\"]))) && isValid(datum[\"mean_precipitation\"]) && isFinite(+datum[\"mean_precipitation\"])"
+        }
+      ]
+    }
+  ],
+  "marks": [
+    {
+      "name": "marks",
+      "type": "rect",
+      "style": ["bar"],
+      "from": {"data": "source_0"},
+      "encode": {
+        "update": {
+          "fill": {"value": "#4c78a8"},
+          "ariaRoleDescription": {"value": "bar"},
+          "description": {
+            "signal": "\"date (month): \" + (timeFormat(datum[\"month_date\"], timeUnitSpecifier([\"month\"], {\"year-month\":\"%b %Y \",\"year-month-date\":\"%b %d, %Y \"}))) + \"; Mean of precipitation: \" + (format(datum[\"mean_precipitation\"], \"\"))"
+          },
+          "x2": {"scale": "x", "field": "month_date", "offset": 3},
+          "x": {"scale": "x", "field": "month_date_end", "offset": -2},
+          "y": {"scale": "y", "field": "mean_precipitation"},
+          "y2": {"scale": "y", "value": 0}
+        }
+      }
+    }
+  ],
+  "scales": [
+    {
+      "name": "x",
+      "type": "time",
+      "domain": {
+        "data": "source_0",
+        "fields": ["month_date", "month_date_end"]
+      },
+      "range": [0, {"signal": "width"}]
+    },
+    {
+      "name": "y",
+      "type": "linear",
+      "domain": {"data": "source_0", "field": "mean_precipitation"},
+      "range": [{"signal": "height"}, 0],
+      "nice": true,
+      "zero": true
+    }
+  ],
+  "axes": [
+    {
+      "scale": "x",
+      "orient": "bottom",
+      "gridScale": "y",
+      "grid": true,
+      "tickMinStep": {
+        "signal": "datetime(2001, 1, 1, 0, 0, 0, 0) - datetime(2001, 0, 1, 0, 0, 0, 0)"
+      },
+      "domain": false,
+      "labels": false,
+      "aria": false,
+      "maxExtent": 0,
+      "minExtent": 0,
+      "ticks": false,
+      "zindex": 0
+    },
+    {
+      "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": "date (month)",
+      "format": {
+        "signal": "timeUnitSpecifier([\"month\"], {\"year-month\":\"%b %Y \",\"year-month-date\":\"%b %d, %Y \"})"
+      },
+      "labelFlush": true,
+      "labelOverlap": true,
+      "tickMinStep": {
+        "signal": "datetime(2001, 1, 1, 0, 0, 0, 0) - datetime(2001, 0, 1, 0, 0, 0, 0)"
+      },
+      "zindex": 0
+    },
+    {
+      "scale": "y",
+      "orient": "left",
+      "grid": false,
+      "title": "Mean of precipitation",
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(height/40)"},
+      "zindex": 0
+    }
+  ]
+}

examples/specs/line_month_center_band_offset.vl.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
+  "data": {"url": "data/seattle-weather.csv"},
+  "mark": {"type": "bar", "xOffset": -2, "x2Offset": 2},
+  "encoding": {
+    "x": {"timeUnit": "month", "field": "date", "type": "temporal"},
+    "y": {"aggregate": "mean", "field": "precipitation"}
+  }
+}

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

@@ -302,6 +302,7 @@ function rectBinPosition({
   const vgChannel2 = getVgPositionChannel(channel2);
 
   const {offset} = positionOffset({channel, markDef, encoding, model, bandPosition: 0});
+  const {offset: offset2} = positionOffset({channel: channel2, markDef, encoding, model, bandPosition: 0});
 
   const bandPosition = isSignalRef(bandSize)
     ? {signal: `(1-${bandSize.signal})/2`}
@@ -315,7 +316,7 @@ function rectBinPosition({
         fieldDef,
         scaleName,
         bandPosition,
-        offset: getBinSpacing(channel2, spacing, reverse, axisTranslate, offset)
+        offset: getBinSpacing(channel2, spacing, reverse, axisTranslate, offset2 ?? offset)
       }),
       [vgChannel]: rectBinRef({
         fieldDef,