Add the example

examples/compiled/stacked_area_without_agg.vg.json

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+{
+  "$schema": "https://vega.github.io/schema/vega/v5.json",
+  "background": "white",
+  "padding": 5,
+  "width": 200,
+  "height": 200,
+  "style": "cell",
+  "data": [
+    {
+      "name": "source_0",
+      "values": [
+        {"a": 1, "b": 2, "c": "c1"},
+        {"a": 2, "b": 3, "c": "c1"},
+        {"a": 3, "b": 1, "c": "c1"},
+        {"a": 1, "b": 2, "c": "c2"},
+        {"a": 3, "b": 1, "c": "c2"},
+        {"a": 2, "b": 2, "c": "c2"}
+      ]
+    },
+    {
+      "name": "data_0",
+      "source": "source_0",
+      "transform": [
+        {"type": "formula", "expr": "toNumber(datum[\"a\"])", "as": "a"}
+      ]
+    }
+  ],
+  "marks": [
+    {
+      "name": "pathgroup",
+      "type": "group",
+      "from": {
+        "facet": {
+          "name": "faceted_path_main",
+          "data": "data_0",
+          "groupby": ["c"]
+        }
+      },
+      "encode": {
+        "update": {
+          "width": {"field": {"group": "width"}},
+          "height": {"field": {"group": "height"}}
+        }
+      },
+      "marks": [
+        {
+          "name": "marks",
+          "type": "area",
+          "style": ["area"],
+          "sort": {"field": "datum[\"a\"]"},
+          "from": {"data": "faceted_path_main"},
+          "encode": {
+            "update": {
+              "orient": {"value": "vertical"},
+              "fill": {"scale": "color", "field": "c"},
+              "description": {
+                "signal": "\"a: \" + (format(datum[\"a\"], \"\")) + \"; b: \" + (format(datum[\"b\"], \"\")) + \"; c: \" + (isValid(datum[\"c\"]) ? datum[\"c\"] : \"\"+datum[\"c\"])"
+              },
+              "x": {"scale": "x", "field": "a"},
+              "y": {"scale": "y", "field": "b"},
+              "y2": {"scale": "y", "value": 0},
+              "defined": {
+                "signal": "isValid(datum[\"a\"]) && isFinite(+datum[\"a\"]) && isValid(datum[\"b\"]) && isFinite(+datum[\"b\"])"
+              }
+            }
+          }
+        }
+      ]
+    }
+  ],
+  "scales": [
+    {
+      "name": "x",
+      "type": "linear",
+      "domain": {"data": "data_0", "field": "a"},
+      "range": [0, {"signal": "width"}],
+      "nice": true,
+      "zero": false
+    },
+    {
+      "name": "y",
+      "type": "linear",
+      "domain": {"data": "data_0", "field": "b"},
+      "range": [{"signal": "height"}, 0],
+      "nice": true,
+      "zero": true
+    },
+    {
+      "name": "color",
+      "type": "ordinal",
+      "domain": {"data": "data_0", "field": "c", "sort": true},
+      "range": "category"
+    }
+  ],
+  "axes": [
+    {
+      "scale": "x",
+      "orient": "bottom",
+      "gridScale": "y",
+      "grid": true,
+      "tickCount": {"signal": "ceil(width/40)"},
+      "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": "a",
+      "labelFlush": true,
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(width/40)"},
+      "zindex": 0
+    },
+    {
+      "scale": "y",
+      "orient": "left",
+      "grid": false,
+      "title": "b",
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(height/40)"},
+      "zindex": 0
+    }
+  ],
+  "legends": [{"fill": "color", "symbolType": "circle", "title": "c"}]
+}

examples/specs/stacked_area_without_agg.vl.json

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+{
+  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
+  "data": {
+    "values": [
+      {"a": 1, "b": 2, "c": "c1"},
+      {"a": 2, "b": 3, "c": "c1"},
+      {"a": 3, "b": 1, "c": "c1"},
+      {"a": 1, "b": 2, "c": "c2"},
+      {"a": 3, "b": 1, "c": "c2"},
+      {"a": 2, "b": 2, "c": "c2"}
+    ]
+  },
+  "mark": "area",
+  "encoding": {
+    "x": {"field": "a", "type": "quantitative"},
+    "y": {"field": "b", "type": "quantitative"},
+    "color": {"field": "c"}
+  }
+}