用于饼图的 vega lite 分组

Question

我如何知道 vega-lite 会产生多少分组？示例用例是：说..我想派生该分组信息以显示一条消息，说“您有太多分组”或根据分组数量找出更好的配色方案。 此处为 vega-lite 规范示例：

{
  "$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

假设您的问题是“我如何计算 category 字段中有多少唯一项目，您可以使用 joinaggregate 转换和 distinct 运算符来计算。

// ... the rest of your spec
"transform": [
    {
      "joinaggregate": [{
       "op": "distinct",
       "field": "categories",
       "as": "number_of_categories"
      }]
    }

在您的示例中，结果将是向表中的每一行添加一个值为 6 的列 number_of_categories。

您可以阅读有关joinaggregate here 的更多信息，并找到所有可用的运算符on the aggregate 页面。