feat: support datum in encoding

[kanitw]

Replaces #4201

Fix #1601

Merge #6149 first!

TODO

• check all

  • isFieldDef

  • getFieldDef

  • model.fieldDef

  • hasConditionalFieldDef

  • channelHasField

  • forEachFieldDef

  • reduceFieldDef

  • support datum for lat/long

  • repeat in datum

Examples

• simple layer datum
• repeat_layer
• layer_bar_annotations

Later

• docs
• Make datum support date time object
• formatSignalRef?
• we should remove ValueDef from lat/long Lat/Long shouldn't support ValueDef #6154
• weird issue with Encoding can't be assigned to Encoding>

[domoritz]

I also just found the code to patch :-P

[domoritz]

We are off the domain line by half a pixel here but I guess that's a known issue.

[kanitw]

We are off the domain line by half a pixel here but I guess that's a known issue.

Yeah, and I'd rather suggest people to use conditional gridColor for the zero baseline. So I think that's not the point of this PR.

[domoritz]

In the example above, we are drawing the rule for each item in the dataset. Can we optimize the spec when there is not field encoding?

[domoritz]

It's actually more than an optimization if we use opacity for the rule.

[domoritz]

Actually, the right spec is

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "layer": [
    {
      "data": {
        "values": [
          {"a": "A", "b": 28},
          {"a": "B", "b": 55},
          {"a": "C", "b": 43},
          {"a": "D", "b": 91},
          {"a": "E", "b": -81},
          {"a": "F", "b": 53},
          {"a": "G", "b": -19},
          {"a": "H", "b": 87},
          {"a": "I", "b": 52}
        ]
      },
      "mark": "line",
      "encoding": {
        "x": {"field": "a", "type": "ordinal"},
        "y": {"field": "b", "type": "quantitative"}
      }
    },
    {"data": {"values": [{}]}, "mark": "rule", "encoding": {"y": {"datum": 0}}}
  ]
}