Allow spec without data source

{
  "$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"}
      }
    },
    {"mark": "rule", "encoding": {"y": {"value": 50}}}
  ]
}

should render the rule layer and the rule layer should have no data source bound in Vega.

[jakevdp]

I think the empty default data is a good idea, though I’d mildly advocate for "data": { "values": [{}] } rather than "data": { "values": [1] }.

[domoritz]

I like that idea as an extension of this issue! Thanks for the suggestion @eflister.