Allow setting default in ordinal range for color

I have a discrete field and I am able to map different colors to it’s different values. However, I would like to have a final color map to all the other colors which I didn’t set up a mapping for.

This might be possible already and I was just unable to find it.

The motiviation is reproducing MapD’s example of plotting dots for each tweet colored by country: https://www.omnisci.com/docs/latest/6_vegaTutorials.html#getting-more-from-your-data-1

In their custom Vega they have a way of specifying a default value:

"scales:" [
     .
     .
     .
  {
    "name": "color",
    "type": "ordinal",
    "domain": ["en",  "es", "fr"],
    "range": ["#27aeef", "#87bc45", "#ef9b20"],
    "default": "gray",
    "nullValue": "#cacaca"
  }
]

[domoritz]

Either way, I don’t think Vega-Lite can do much here since it doesn’t have access to the data. Should I move the issue to Vega?

[kanitw]

Also: Under the hood, D3 does support setting an unknown value on ordinal scales, however, this will return a default value for any value not included in the domain, not just null or missing data. One option for Vega could be to leverage that functionality, so long as those semantics are acceptable.

I think this semantic is okay for handling the “default/unknown/else” case. It would only be useful when users know a few values that they care about like ["en", "es", "fr"]. But for data-driven domain (default in VL), the unknown case wouldn’t happen.

scale('color', datum.category) || 'grey' still doesn’t work for handling null value in Vega-Lite though, since Vega-Lite uses data-driven domain. Thus, if the data includes null (or undefined), they will still be included as one of the domain values.

We can sort of get around this using a conditional rule. However, null wouldn’t then be included in the legend.

If we want to include null in the legend, then we still need to do something more.

a) If there is a way to combine range, then we can do something like:

"domain": {"fields": [[null],{"data": "source_0", "field": "Major_Genre", "sort": true}]},
"range": // some syntax to concatenate grey with scheme 

but it won’t work for undefined.

b) Another solution would be simply adding a legend without title with only null value and just its color. (This probably also works for null/invalid value of quantitative/sequential color scale.) The trick part is then showing this legend only when the data field contains null.