Legend loses gradient if shared between layers

The following:

{
  "layer": [{
    "mark": { "type": "line" },
    "encoding": {
      "color": {
        "type": "quantitative",
        "field": "c"
      }
    }
  }, {
    "mark": { "type": "circle" },
    "encoding": {
      "color": {
        "type": "quantitative",
        "field": "c"
      }
    }
  }],
  "data": {"values": [{"c": 1}, {"c": 2}]}
}

Currently produces:

This is surprising - I would have expected a gradient legend.

If a layer is removed (which one is removed doesn’t matter), the output is correct:

[kanitw]

I don’t think this is expected behavior

I think we could fix this in a few ways:

1. in Vega (when fill and stroke encodes the same color). or 2) encode “color” in legend parse and convert it to fill or stroke later in assemble.

That said, since there is a good workaround, I’ll make this issue P3.

[domoritz]

Okay, so the issue is that Vega uses a different legend when both fill and stroke are encoding a scale. That’s a bit surprising but also kinda makes sense since only a symbol can encode both gradient and fill.

Removing one or the other does not make sense in the general case so I think that the behavior is actually as expected.

If you want a gradient legend, you need to tell Vega-Lite with "legend": {"type": "gradient"}.