`legend.type: "symbol"` dropped from compiled Vega for discretizing color scales (threshold/quantize/quantile/bin)

[shenker]

Vega-Lite: 6.4 (also repros on 5.x)
Vega: 6.2.0

Description

For a quantitative color encoding with a discretizing scale (threshold, quantize, quantile, or bin), legend.type: "symbol" is silently dropped during vega-lite to Vega compilation. The compiled Vega legend has no type, so Vega renders a gradient legend instead of the requested symbol legend.

A linear color scale with the same legend.type: "symbol" does propagate correctly — so the property is supported, just not applied for these scale types.

Reproduction

Paste into the Vega Editor:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.4.1.json",
  "config": {
    "view": {
      "continuousHeight": 300,
      "continuousWidth": 300
    }
  },
  "data": {
    "format": {
      "feature": "counties",
      "type": "topojson"
    },
    "url": "https://cdn.jsdelivr.net/npm/vega-datasets@v3.2.1/data/us-10m.json"
  },
  "encoding": {
    "color": {
      "field": "rate",
      "legend": {
        "type": "symbol"
      },
      "scale": {
        "domain": [
          0.1,
          0.2,
          0.3
        ],
        "type": "threshold"
      },
      "type": "quantitative"
    }
  },
  "height": 300,
  "mark": {
    "type": "geoshape"
  },
  "projection": {
    "type": "albersUsa"
  },
  "transform": [
    {
      "from": {
        "data": {
          "url": "https://cdn.jsdelivr.net/npm/vega-datasets@v3.2.1/data/unemployment.tsv"
        },
        "fields": [
          "rate"
        ],
        "key": "id"
      },
      "lookup": "id"
    }
  ],
  "usermeta": {
    "embedOptions": {}
  },
  "width": 500
}

Expected vs. Actual (compiled Vega legend)

Expected: In the compiled Vega,

"legends": [
    {
      "fill": "color",
      "labelOverlap": "greedy",
      "symbolType": "circle",
      "title": "rate",
      "type": "symbol"
    }
  ],

Actual:

"legends": [
    {
      "fill": "color",
      "labelOverlap": "greedy",
      "symbolType": "circle",
      "title": "rate"
    }
  ],

Manually adding "type": "symbol" to this Vega legend produces the intended discrete legend.

[shenker]

Did a bit more digging. This fixes the issue, though I'll leave the PR to someone who knows the vega-lite codebase.

diff --git a/src/compile/legend/properties.ts b/src/compile/legend/properties.ts
index ad6e885..f0300b4 100644
--- a/src/compile/legend/properties.ts
+++ b/src/compile/legend/properties.ts
@@ -66,12 +66,17 @@ export const legendRules: {
 
   title: ({fieldOrDatumDef, config}) => fieldDefTitle(fieldOrDatumDef, config, {allowDisabling: true}),
 
-  type: ({legendType, scaleType, channel}) => {
+  type: ({legend, legendType, scaleType, channel}) => {
     if (isColorChannel(channel) && isContinuousToContinuous(scaleType)) {
       if (legendType === 'gradient') {
         return undefined;
       }
-    } else if (legendType === 'symbol') {
+    } else if (legendType === 'symbol' && legend.type === undefined) {
       return undefined;
     }
     return legendType;

[aloktomarr]

@shenker

Thanks for the detailed diagnosis!

The type rule in src/compile/legend/properties.ts was dropping symbol for all non-continuous-color cases (assuming it's Vega's default), but Vega defaults discretizing color scales to gradient, so an explicit legend.type: "symbol" got silently dropped.

I've got a fix that keeps the type when it's explicitly set (only the inferred symbol is omitted), with tests. Opened a PR.