Format of null data in tooltips.

[PhML]

In the following example, I use a tooltip to display underlying data but I have troubles to have consistent behavior when displaying null values of different types.

Indeed, while missing quantitative data leads to display NaN (no c key in third value of values), temporal data is interpreted as zero (Jan 01, 1970 00:00:00). I expect to display null values for temporal data as null or NaT to be consistent with NaN display of quantitative data.

When values are explicitly null, behavior is consistent as both quantitative and temporal data are rendered as respectively 0 and Jan 01, 1970 00:00:00.

But I rather like to display null or N/A in such case. So having a way to specify how to handle null values. (Please, tell me how if it is possible).

{
  "data": {
    "values": [
      {
        "a": "A",
        "b": 28,
        "c": null,
        "d": null
      },
      {
        "a": "B",
        "b": 55,
        "c": "2020-04-27T14:15:18.586949+00:00",
        "d": 1000
      },
      {
        "a": "C",
        "b": 43
      }
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {
      "field": "a",
      "type": "ordinal"
    },
    "y": {
      "field": "b",
      "type": "quantitative"
    },
    "tooltip": [
      {
        "field": "a",
        "type": "ordinal"
      },
      {
        "field": "b",
        "type": "quantitative"
      },
      {
        "field": "c",
        "type": "temporal",
        "title": "c",
        "timeUnit": "yearmonthdatehoursminutesseconds",
        "scale": {
          "type": "utc"
        }
      },
      {
        "field": "d",
        "type": "quantitative"
      }
    ]
  }
}

Open the Chart in the Vega Editor

[kanitw]

We need to adjust formatSignalRef to handle this consistently, while respecting config.mark.invalid.

[kanitw]

We also see this issue in https://vega.github.io/editor/#/examples/vega-lite/point_invalid_color

cc: @yhoonkim