How to create a legend in a Deneb Vega-lite visual with multiple layers?

[Meagan]

I have created a Deneb visual that concatenates a view with a text mark on top of a bullet chart/gauge. 

I want to create a legend that explains what the 2 bars and the tick mark mean. But I'm not sure how/where to include it in the visual. I need the symbols on the legend to match the colors in the visual. If I can't make one "natively", I am fine getting creative and concatenating a "fake" one on the bottom. I'd rather it be inside the Deneb visual rather than layering another Power BI visual on top of it. But I'm struggling to figure out the best way to handle this. Vega-lite code is below. 

{
"data": {"name": "dataset"},
"vconcat": [
{
"mark": {
"type": "text",
"fontSize": 40,
"align": "left",
"baseline": "top",
"dy": 20,
"fontWeight": "bold"
},
"encoding": {
"text": {
"field": "Actual",
"type": "quantitative",
"format": "$,.0f"
},
"color": {"expr": "pbicolor(3)"}
}
},
{ "width": "300",
"encoding": {
"x": {
"type": "quantitative",
"scale": {"nice": false},
"title": null,
"axis": null
},
"y": {"type": "nominal"},
"tooltip": [
{"field": "Actual"},
{"field": "Budget"},
{"field": "Projection"}
]
},
"layer": [
{
"mark": {
"type": "bar",
"cornerRadiusEnd": 8,
"color": {
"expr": "pbiColor(4, .6)"
},
"size": 60
},
"encoding": {
"x": {"field": "Budget"}
}
},
{
"mark": {
"type": "bar",
"cornerRadiusEnd": 8,
"color": {
"expr": "pbiColor(4)"
},
"size": 40
},
"encoding": {
"x": {"field": "Actual"}
}
},
{
"mark": {
"type": "tick",
"height": 70,
"thickness": 5,
"color": {
"expr": "pbiColor(4,-.5)"
}
},
"encoding": {
"x": {"field": "Projection"}
}
}
]
}
]
}

[Meagan · Accepted solution]

Ok, I got it working following this custom legend tutorial from Enterprise DNA: https://forum.enterprisedna.co/t/deneb-examples-legends/28900. I don't love that I had to make a fake chart, but it does work well. Basically add another view to the vconcat, make a fake chart using an arc marc with a radius of zero. Hard code the legend values in the data for that view.