Vega Lite: Normalized Stacked Bar Chart + Overlay percentages as text - vega-lite

Question

I have a stacked normalized bar chart similar to this:
https://vega.github.io/editor/#/examples/vega-lite/stacked_bar_normalize
I'm trying to show the related percentages (per bar segment) as text on the bars similar to: https://gist.github.com/pratapvardhan/00800a4981d43a84efdba0c4cf8ee2e1
I tried adding a transform field to calculate the percentages, but still couldn't get it to work after hours of trying.
I'm lost help 🥺
My best try:
{
"description":
"A bar chart showing the US population distribution of age groups and gender in 2000.",
"data": {
"url": "data/population.json"
},
"transform": [
{"filter": "datum.year == 2000"},
{"calculate": "datum.sex == 2 ? 'Female' : 'Male'", "as": "gender"},
{
"stack": "people",
"offset": "normalize",
"as": ["v1", "v2"],
"groupby": ["age"],
"sort": [{"field": "gender", "order": "descending"}]
}
],
"encoding": {
"y": {
"field": "v1",
"type": "quantitative",
"title": "population"
},
"y2": {"field": "v2"},
"x": {
"field": "age",
"type": "ordinal"
},
"color": {
"field": "gender",
"type": "nominal",
"scale": {
"range": ["#675193", "#ca8861"]
}
}
},
"layer":[
{ "mark": "bar"},
{"mark": {"type": "text", "dx": 0, "dy": 0},
"encoding": {
"color":{"value":"black"},
"text": { "field": "v1", "type": "quantitative", "format": ".1f"}}
}
]
}

Answer 1

You can use a joinaggregate transform to normalize each group, and then use "format": ".1%" to display fractions as percents. Using this, there is no need to manually compute the stack transform; it is simpler to specify the stack via the encoding, as in the example you linked to.
Here is the result (open in editor):
{
"description": "A bar chart showing the US population distribution of age groups and gender in 2000.",
"data": {"url": "data/population.json"},
"transform": [
{"filter": "datum.year == 2000"},
{"calculate": "datum.sex == 2 ? 'Female' : 'Male'", "as": "gender"},
{
"joinaggregate": [{"op": "sum", "field": "people", "as": "total"}],
"groupby": ["age"]
},
{"calculate": "datum.people / datum.total", "as": "fraction"}
],
"encoding": {
"y": {
"aggregate": "sum",
"field": "people",
"title": "population",
"stack": "normalize"
},
"order": {"field": "gender", "sort": "descending"},
"x": {"field": "age", "type": "ordinal"},
"color": {
"field": "gender",
"type": "nominal",
"scale": {"range": ["#675193", "#ca8861"]}
}
},
"layer": [
{"mark": "bar"},
{
"mark": {"type": "text", "dx": 20, "dy": 0, "angle": 90},
"encoding": {
"color": {"value": "white"},
"text": {"field": "fraction", "type": "quantitative", "format": ".1%"}
}
}
]
}