Drop color and opacity encodings for trail marks

[armsp]

Not sure if it is a bug or an enhancement.

I am using Trail to make a line chart with a varying size as well as opacity. I added the opacity encoding, which does not give any errors, but the graph itself vanishes.

Following is a very simple example of what I want to do -

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "values": [
      {"a": "A", "b": 0.28}, {"a": "B", "b": 0.55}, {"a": "C", "b": 0.43},
      {"a": "D", "b": 0.91}, {"a": "E", "b": 0.81}, {"a": "F", "b": 0.53},
      {"a": "G", "b": 0.19}, {"a": "H", "b": 0.87}, {"a": "I", "b": 0.52}
    ]
  },
  "mark": "trail",
  "encoding": {
    "x": {"field": "a", "type": "ordinal"},
    "y": {"field": "b", "type": "quantitative"},
    "size": {"field": "b", "type": "quantitative"},
    "opacity":{"field": "b",  "type": "quantitative"}
  }
}

Is it possible in Vega-Lite?

[domoritz]

That looks like a bug. Interestingly, value works.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "values": [
      {"a": "A", "b": 0.28}, {"a": "B", "b": 0.55}, {"a": "C", "b": 0.43},
      {"a": "D", "b": 0.91}, {"a": "E", "b": 0.81}, {"a": "F", "b": 0.53},
      {"a": "G", "b": 0.19}, {"a": "H", "b": 0.87}, {"a": "I", "b": 0.52}
    ]
  },
  "mark": "trail",
  "encoding": {
    "x": {"field": "a", "type": "ordinal"},
    "y": {"field": "b", "type": "quantitative"},
    "size": {"field": "b", "type": "quantitative"},
    "opacity":{"value": 0.2}
  }
}

I don't yet know whether Vega supports different opacities for different sections.

As a workaround, use color instead of opacity for now.

[domoritz]

Ups, color is also not working. Open the Chart in the Vega Editor

Does trail support mutiple colors?

[domoritz]

Actually, this is expected behavior as the trail mark only supports a single color and opacity. I guess we should drop encodings that are not supported to trail marks. We should show a warning. We should still support value encodings.

[armsp]

@domoritz So is it not possible to get varying opacity values in Vega-Lite, perhaps by using any other mark or technique/hack ?

[domoritz]

I think so. You could add many line segments each with a different color but you won't get a continuous color change.

[jwoLondon]

Perhaps its a Vega rather than Vega-Lite issue, but providing full channel encoding for trails would be really useful. I'd love to see line interpolation working too (monotone, basis etc.).

My use case: arrow-like symbolisation on flow maps. Currently in Vega-Lite I can create straight arrows with trails, or curved lines with line marks, but not curved trails of varying thickness (trails always use linear interpolation for positioning of vertices).