Vega-Lite: How do I include image marks in a doughnut chart? - vega-lite

Question

I would like to have image marks surrounding my doughnut chart instead of texts. The example for image marks use x and y for its coordinate. How should I adjust that for a doughnut chart where we work with radius and theta?
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"description": "A simple pie chart with labels.",
"data": {
"values": [
{"category": "a", "value": 4, "image": url},
{"category": "b", "value": 6, "image": url},
{"category": "c", "value": 10, "image": url},
{"category": "d", "value": 3, "image": url},
{"category": "e", "value": 7, "image": url},
{"category": "f", "value": 8, "image": url}
]
},
"encoding": {
"theta": {"field": "value", "type": "quantitative", "stack": true},
"color": {"field": "category", "type": "nominal", "legend": null}
},
"layer": [{
"mark": {"type": "arc", "outerRadius": 80}
}, {
"mark": {"type": "text", "radius": 90},
"encoding": {
"text": {"field": "category", "type": "nominal"}
}
}],
"view": {"stroke": null}
}
New vega version:
Open the Chart in the Vega Editor

Answer 1

After some trials and reading through the doc, it seems Image Mark cannot be positioned by theta encoding, but the example shows that x and y encodings are supported.
Therefore, I worked out this positioning via simple trigonometry and an extra layer to place the images in the doughnut:
{
"transform": [
{"joinaggregate": [{"op":"sum", "field": "value", "as": "total"}]},
{
"window": [{"op": "sum", "field": "value", "as": "cum"}],
"frame": [null, 0]
},
{"calculate": "cos(2*PI*(datum.cum-datum.value/2)/datum.total)", "as": "y"},
{"calculate": "sin(2*PI*(datum.cum-datum.value/2)/datum.total)", "as": "x"}
],
"mark": {"type": "image", "width": 20, "height": 20},
"encoding": {
"url": {"field": "image"},
"x": {"field": "x", "type": "quantitative", "scale": {"domain": [-2, 2]}, "axis": null},
"y": {"field": "y", "type": "quantitative", "scale": {"domain": [-2, 2]}, "axis": null}
}
}
Yet another Vega Editor
As the order is messed up by the color encoding mentioned in comments below, a new window transform is added to generate an extra ordering field which is provided to color field
Renewed Vega Editor
3 changes were made: (2021-07-16)
Using cos in the calculate of y
Using sin in the calculate of x
Messing up the data value to check if working
Old & Wrong Vega Editor