Invalid scale reference causes domain pollution (for multi-field data ref domain)

In the following spec, the fill spec for the right axis causes an undefined value to appear in the color scale’s domain. Verified in the REPL by viewing the rendered legend and also by logging VEGA_DEBUG.view._runtime.scales.color.value.domain() (returns ["A", "B", "val", undefined]).

Removing the right axis fill spec fixes the problem. The fill spec in question is invalid, as the axis encoding must map to a single color; the same spec used e.g. within a mark encoding does not trigger this issue.

Not a problem for me now that I understand I have an incorrect usage I need to fix, but it took me a while to figure out why my domain was rendering an undefined value…

{
  "$schema": "https://vega.github.io/schema/vega/v3.0.json",
  "width": 400,
  "height": 300,
  "data": [
    {
      "name": "dataset",
      "values": [
        {"bin": "A", "val": 10},
        {"bin": "B", "val": 20},
        {"bin": "A", "val": 30},
        {"bin": "B", "val": 40}
      ]
    }
  ],
  "scales": [
    {
      "name": "y",
      "type": "linear",
      "domain": {
        "data": "dataset",
        "field": "y_out"
      },
      "zero": true,
      "range": "height"
    },
    {
      "name": "color",
      "type": "ordinal",
      "domain": {
        "fields": [
          {"data": "dataset", "field": "bin"},
          ["val"]
        ]
      },
      "range": ["#DDB27C", "#11939a", "#11939a"]
    }
  ],
  "axes": [
    {
      "orient": "left",
      "scale": "y",
      "title": "left side",
      "encode": {
        "title": {
          "update": {
            "fill": {
              "signal": "scale('color', 'val')"
            }
          }
        }
      }
    },
    {
      "orient": "right",
      "scale": "y",
      "title": "right side",
      "encode": {
        "title": {
          "update": {
            "fill": {
              "scale": "color",
              "field": "bin"
            }
          }
        }
      }
    }
  ],
  "legends": [
    {
      "fill": "color"
    }
  ]
}

[ericsoco]

Ha, I win by one second 😉

[jheer]

Right now, one can define an ordinal scale with a range (e.g., category10) and no corresponding domain definition. The domain is then populated as values are passed through the scale function. As we have supported this since Vega 1, I’d be hesitant to break this feature. On the other hand, if a domain is explicitly defined within the spec, it seems reasonable to me to disable automatic domain extension when new values are encountered.