vega lite grouping for pie chart - vega-lite

Question

how would i find out how many grouping will result from vega-lite? Sample use cases would be: say .. i want to derive that grouping information to maybe display a message that say "you have too many groupings" or figure out a better color scheme based on number of grouping.
sample vega-lite spec here:
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"description": "A simple pie chart with labels.",
"data": {
"values": [
{"category": "a", "value": 4},
{"category": "b", "value": 6},
{"category": "c", "value": 10},
{"category": "d", "value": 3},
{"category": "e", "value": 7},
{"category": "f", "value": 8},{"category": "f", "value": 8}
]
},
"encoding": {
"theta": {"field": "value", "type": "quantitative", "stack": true},
"color": {"field": "category", "type": "nominal"}
},
"layer": [{
"mark": {"type": "arc", "outerRadius": 80}
}, {
"mark": {"type": "text", "radius": 90},
"encoding": {
"text": {"field": "category", "type": "nominal"}
}
}]
}

Answer 1

Assuming your question is "how can I calculate how many unique items are in the category field, you could use joinaggregate transform with the distinct operator to count.
// ... the rest of your spec
"transform": [
{
"joinaggregate": [{
"op": "distinct",
"field": "categories",
"as": "number_of_categories"
}]
}
In your example, the result would be to add a column number_of_categories with a value of 6 to every row in your table.
You can read more about joinaggregate here, and find all the available operators on the aggregate page.