zindex in encoding channels not working with signals nor conditions #4065
Description
When setting zindex in an encoding channel, it doesn't seem to respect signals nor conditional values via the [{test}] syntax.
See this example, where a signal for the line mark correctly colors 2 different lines, but the same signal on zindex does not change the ordering: Vega Editor example
{
"$schema": "https://vega.github.io/schema/vega/v5.json",
"description": "A basic line chart example.",
"width": 500,
"height": 200,
"padding": 5,
"signals": [
{
"name": "interpolate",
"value": "linear",
"bind": {
"input": "select",
"options": [
"basis",
"cardinal",
"catmull-rom",
"linear",
"monotone",
"natural",
"step",
"step-after",
"step-before"
]
}
}
],
"data": [
{
"name": "table",
"values": [
{"x": 0, "y": 28, "c":0}, {"x": 0, "y": 20, "c":1},
{"x": 1, "y": 43, "c":0}, {"x": 1, "y": 35, "c":1},
{"x": 2, "y": 81, "c":0}, {"x": 2, "y": 10, "c":1},
{"x": 3, "y": 19, "c":0}, {"x": 3, "y": 15, "c":1},
{"x": 4, "y": 52, "c":0}, {"x": 4, "y": 48, "c":1},
{"x": 5, "y": 24, "c":0}, {"x": 5, "y": 28, "c":1},
{"x": 6, "y": 87, "c":0}, {"x": 6, "y": 66, "c":1},
{"x": 7, "y": 17, "c":0}, {"x": 7, "y": 27, "c":1},
{"x": 8, "y": 68, "c":0}, {"x": 8, "y": 16, "c":1},
{"x": 9, "y": 49, "c":0}, {"x": 9, "y": 25, "c":1}
]
}
],
"scales": [
{
"name": "x",
"type": "point",
"range": "width",
"domain": {"data": "table", "field": "x"}
},
{
"name": "y",
"type": "linear",
"range": "height",
"nice": true,
"zero": true,
"domain": {"data": "table", "field": "y"}
},
{
"name": "color",
"type": "ordinal",
"range": "category",
"domain": {"data": "table", "field": "c"}
}
],
"axes": [
{"orient": "bottom", "scale": "x"},
{"orient": "left", "scale": "y"}
],
"marks": [
{
"type": "group",
"from": {
"facet": {
"name": "series",
"data": "table",
"groupby": "c"
}
},
"marks": [
{
"type": "line",
"from": {"data": "series"},
"encode": {
"enter": {
"x": {"scale": "x", "field": "x"},
"y": {"scale": "y", "field": "y"},
"stroke": {"scale": "color", "field": "c"},
"strokeWidth": {"value": 8}
},
"update": {
"interpolate": {"signal": "interpolate"},
// this works
"stroke": {"signal": "datum.c === 0 ? 'blue' : 'red'"},
// this doesn't work? the blue line (c=0) should appear in front
"zindex": { "signal": "datum.c === 0? 1 : 0"}
},
}
}
]
}
]
}