Vega-lite: Encoding's `datum` for constant values in data domain that still pass through scales

[kanitw]

value in fieldDef set the range value directly.

If users wanna provide domain value that needs to be scaled, we need to add another property.
From a conversation long time ago, we decided that this property is datum.

To clarify, a value property in VL produces a Vega ValueRef {value: value}.
Meanwhile, a datum property in VL should produces a Vega Value Ref {scale:..., value: datum} and use scale from relevant layers.

cc: @swojit

[kanitw]

• [x] Change Dropping condition in unit.ts

[kanitw]

With this feature, the spec in https://github.com/vega/vega-lite/issues/3203#issuecomment-345422152 could become:

{
  "data": {"url": "data/stocks.csv"},
  "layer": [
    {
      "mark": "line",
      "encoding": {
        "x": {"field": "date", "type": "temporal"},
        "y": {"field": "price", "type": "quantitative"},
        "color": {"field": "symbol", "type": "nominal"}
      }
    },
    {
      "mark": "rule",
      "encoding": {
        "y": { "datum": 100},
        "size": {"value": 2}
      }
    }
  ]
}

[kanitw]

Since this is just a syntactic sugar and there is already a good workaround, I don't think we will get to do it soon. Just in case any one wanna help, this would involve:

1) Add a new DatumDef as a new type of ChannelDef
2) Modify scale creation logic in parseUnitScaleCore in compile/scale/parse.ts to also create scales for datum.
3) Fix all the compile error due to new type of ChannelDef as DatumDef. This should include

• rules for creating scales
• rules in compile/mark/valueref.ts and compile/mark/mixins.ts for how to apply datum with scale. (This part should be quite straightforward)

4) The complicated part is deciding how we deal with axes and legends for datum.

[kanitw]

Examples

• [x] https://github.com/vega/vega-lite/issues/3411 shows another use case where this can be useful for forcing color legend to show up for layered plots. (See https://github.com/vega/vega-lite/issues/1274.)
• [x] Update https://vega.github.io/vega-lite/examples/layer_bar_annotations.html

[davidanthoff]

Just want to add a voice of support for this feature. Yes, there is a work around, but it seems to me that this is a very common use case (at least it came up very quickly when some students in my lab started to use Vega-Lite) so it would be great if there was a smooth way to handle this, as this issue proposes. It seems like one of those things that really ought to be dead simple :)

[domoritz]

Just trying to refresh my brain about this. Why did we decide to add a datum property rather than {scale: null} or {scale: identity}?

[domoritz]

Never mind: https://github.com/vega/vega-lite/pull/3511#issuecomment-372148754.

[kanitw]

Will be fixed in https://github.com/vega/vega-lite/pull/6150