feat: support datum in encoding

examples/compiled/layer_bar_annotations.vg.json

@@ -26,10 +26,7 @@
         {"Day": 15, "Value": 130.5}
       ]
     },
-    {
-      "name": "source_1",
-      "values": [{"ThresholdValue": 300, "Threshold": "hazardous"}]
-    },
+    {"name": "source_1", "values": [{}]},
     {
       "name": "data_1",
       "source": "source_0",
@@ -51,26 +48,6 @@
           "expr": "isValid(datum[\"baseline\"]) && isFinite(+datum[\"baseline\"])"
         }
       ]
-    },
-    {
-      "name": "data_4",
-      "source": "source_1",
-      "transform": [
-        {
-          "type": "filter",
-          "expr": "isValid(datum[\"ThresholdValue\"]) && isFinite(+datum[\"ThresholdValue\"])"
-        }
-      ]
-    },
-    {
-      "name": "data_5",
-      "source": "source_1",
-      "transform": [
-        {
-          "type": "filter",
-          "expr": "isValid(datum[\"ThresholdValue\"]) && isFinite(+datum[\"ThresholdValue\"])"
-        }
-      ]
     }
   ],
   "signals": [
@@ -115,31 +92,31 @@
       "name": "layer_1_layer_0_marks",
       "type": "rule",
       "style": ["rule"],
-      "from": {"data": "data_4"},
+      "from": {"data": "source_1"},
       "encode": {
         "update": {
           "stroke": {"value": "black"},
           "x": {"field": {"group": "width"}},
           "x2": {"value": 0},
-          "y": {"scale": "y", "field": "ThresholdValue"}
+          "y": {"scale": "y", "value": 300}
         }
       }
     },
     {
       "name": "layer_1_layer_1_marks",
       "type": "text",
       "style": ["text"],
-      "from": {"data": "data_5"},
+      "from": {"data": "source_1"},
       "encode": {
         "update": {
           "align": {"value": "right"},
           "baseline": {"value": "bottom"},
+          "text": {"value": "hazardous"},
           "dx": {"value": -2},
           "dy": {"value": -2},
           "fill": {"value": "black"},
           "x": {"field": {"group": "width"}},
-          "y": {"scale": "y", "field": "ThresholdValue"},
-          "text": {"signal": "''+datum[\"Threshold\"]"}
+          "y": {"scale": "y", "value": 300}
         }
       }
     }
@@ -167,8 +144,7 @@
           {"data": "data_1", "field": "Value"},
           {"data": "data_2", "field": "baseline"},
           {"data": "data_2", "field": "Value"},
-          {"data": "data_4", "field": "ThresholdValue"},
-          {"data": "data_5", "field": "ThresholdValue"}
+          [300]
         ]
       },
       "range": [{"signal": "height"}, 0],

examples/compiled/layer_line_datum_rule.vg.json

@@ -0,0 +1,140 @@
+{
+  "$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/stocks.csv",
+      "format": {"type": "csv", "parse": {"date": "date"}}
+    },
+    {"name": "source_1", "values": [{}]}
+  ],
+  "marks": [
+    {
+      "name": "layer_0_pathgroup",
+      "type": "group",
+      "from": {
+        "facet": {
+          "name": "faceted_path_layer_0_main",
+          "data": "source_0",
+          "groupby": ["symbol"]
+        }
+      },
+      "encode": {
+        "update": {
+          "width": {"field": {"group": "width"}},
+          "height": {"field": {"group": "height"}}
+        }
+      },
+      "marks": [
+        {
+          "name": "layer_0_marks",
+          "type": "line",
+          "style": ["line"],
+          "sort": {"field": "datum[\"date\"]"},
+          "from": {"data": "faceted_path_layer_0_main"},
+          "encode": {
+            "update": {
+              "stroke": {"scale": "color", "field": "symbol"},
+              "x": {"scale": "x", "field": "date"},
+              "y": {"scale": "y", "field": "price"},
+              "defined": {
+                "signal": "isValid(datum[\"date\"]) && isFinite(+datum[\"date\"]) && isValid(datum[\"price\"]) && isFinite(+datum[\"price\"])"
+              }
+            }
+          }
+        }
+      ]
+    },
+    {
+      "name": "layer_1_marks",
+      "type": "rule",
+      "style": ["rule"],
+      "from": {"data": "source_1"},
+      "encode": {
+        "update": {
+          "strokeDash": {"value": [2, 2]},
+          "stroke": {"value": "black"},
+          "x": {"field": {"group": "width"}},
+          "x2": {"value": 0},
+          "y": {"scale": "y", "value": 300},
+          "strokeWidth": {"value": 2}
+        }
+      }
+    }
+  ],
+  "scales": [
+    {
+      "name": "x",
+      "type": "time",
+      "domain": {"data": "source_0", "field": "date"},
+      "range": [0, {"signal": "width"}]
+    },
+    {
+      "name": "y",
+      "type": "linear",
+      "domain": {"fields": [{"data": "source_0", "field": "price"}, [300]]},
+      "range": [{"signal": "height"}, 0],
+      "nice": true,
+      "zero": true
+    },
+    {
+      "name": "color",
+      "type": "ordinal",
+      "domain": {"data": "source_0", "field": "symbol", "sort": true},
+      "range": "category"
+    }
+  ],
+  "axes": [
+    {
+      "scale": "x",
+      "orient": "bottom",
+      "gridScale": "y",
+      "grid": true,
+      "tickCount": {"signal": "ceil(width/40)"},
+      "domain": false,
+      "labels": 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,
+      "maxExtent": 0,
+      "minExtent": 0,
+      "ticks": false,
+      "zindex": 0
+    },
+    {
+      "scale": "x",
+      "orient": "bottom",
+      "grid": false,
+      "title": "date",
+      "labelFlush": true,
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(width/40)"},
+      "zindex": 0
+    },
+    {
+      "scale": "y",
+      "orient": "left",
+      "grid": false,
+      "title": "price",
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(height/40)"},
+      "zindex": 0
+    }
+  ],
+  "legends": [{"stroke": "color", "symbolType": "stroke", "title": "symbol"}]
+}

examples/compiled/repeat_layer.vg.json

@@ -51,7 +51,7 @@
       "from": {"data": "source_0"},
       "encode": {
         "update": {
-          "stroke": {"value": "#4c78a8"},
+          "stroke": {"scale": "color", "value": "US_Gross"},
           "x": {
             "signal": "scale(\"x\", 0.5 * datum[\"bin_maxbins_10_IMDB_Rating\"] + 0.5 * datum[\"bin_maxbins_10_IMDB_Rating_end\"])"
           },
@@ -70,7 +70,7 @@
       "from": {"data": "source_0"},
       "encode": {
         "update": {
-          "stroke": {"value": "#4c78a8"},
+          "stroke": {"scale": "color", "value": "Worldwide_Gross"},
           "x": {
             "signal": "scale(\"x\", 0.5 * datum[\"bin_maxbins_10_IMDB_Rating\"] + 0.5 * datum[\"bin_maxbins_10_IMDB_Rating_end\"])"
           },
@@ -105,6 +105,12 @@
       "range": [{"signal": "height"}, 0],
       "nice": true,
       "zero": true
+    },
+    {
+      "name": "color",
+      "type": "ordinal",
+      "domain": {"fields": [["US_Gross"], ["Worldwide_Gross"]]},
+      "range": "category"
     }
   ],
   "axes": [
@@ -135,10 +141,11 @@
       "scale": "y",
       "orient": "left",
       "grid": false,
-      "title": "Mean of US_Gross, Mean of Worldwide_Gross",
+      "title": "Mean of US and Worldwide Gross",
       "labelOverlap": true,
       "tickCount": {"signal": "ceil(height/40)"},
       "zindex": 0
     }
-  ]
+  ],
+  "legends": [{"stroke": "color", "symbolType": "stroke"}]
 }

examples/specs/geo_constant_value.vl.json

@@ -32,7 +32,7 @@
       "mark": "square",
       "encoding": {
         "longitude": {
-          "value": -122.335167
+          "datum": -122.335167
         },
         "latitude": {
           "field": "latitude",
@@ -54,7 +54,7 @@
           "type": "quantitative"
         },
         "latitude": {
-          "value": 47.608013
+          "datum": 47.608013
         },
         "size": {
           "value": 1

examples/specs/layer_bar_annotations.vl.json

@@ -35,40 +35,29 @@
         ],
         "encoding": {
           "x": {"field": "Day", "type": "ordinal"},
-          "y": {"field": "baseline", "type": "quantitative"},
+          "y": {"field": "baseline", "type": "quantitative", "title": "PM2.5 Value"},
           "y2": {"field": "Value"},
           "color": {"value": "#e45755"}
         }
       }
     ]}, {
       "data": {
-         "values": [
-          {"ThresholdValue": 300, "Threshold": "hazardous"}
-        ]
+         "values": [{}]
+      },
+      "encoding": {
+        "y": {"datum": 300}
       },
       "layer": [{
-        "mark": "rule",
-        "encoding": {
-          "y": {"field": "ThresholdValue", "type": "quantitative"}
-        }
+        "mark": "rule"
       }, {
         "mark": {
           "type": "text",
           "align": "right",
           "baseline": "bottom",
           "dx": -2,
-          "dy": -2
-        },
-        "encoding": {
-          "x": {
-            "value": "width"
-          },
-          "y": {
-            "field": "ThresholdValue",
-            "type": "quantitative",
-            "axis": {"title": "PM2.5 Value"}
-          },
-          "text": {"field": "Threshold", "type": "ordinal"}
+          "dy": -2,
+          "x": "width",
+          "text": "hazardous"
         }
       }]
     }

examples/specs/layer_line_datum_rule.vl.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
+  "layer": [
+    {
+      "data": {"url": "data/stocks.csv"},
+      "mark": "line",
+      "encoding": {
+        "x": {"field": "date", "type": "temporal"},
+        "y": {"field": "price", "type": "quantitative"},
+        "color": {"field": "symbol", "type": "nominal"}
+      }
+    },
+    {
+      "data": {"values": [{}]},
+      "mark": {"type": "rule", "strokeDash": [2, 2], "size": 2},
+      "encoding": {
+        "y": {"datum": 300}
+      }
+    }
+  ]
+}

examples/specs/normalized/geo_constant_value_normalized.vl.json

@@ -17,7 +17,7 @@
     {
       "mark": "square",
       "encoding": {
-        "longitude": {"value": -122.335167},
+        "longitude": {"datum": -122.335167},
         "latitude": {"field": "latitude", "type": "quantitative"},
         "size": {"value": 1},
         "color": {"value": "steelblue"}
@@ -28,7 +28,7 @@
       "mark": "square",
       "encoding": {
         "longitude": {"field": "longitude", "type": "quantitative"},
-        "latitude": {"value": 47.608013},
+        "latitude": {"datum": 47.608013},
         "size": {"value": 1},
         "color": {"value": "firebrick"}
       },