vega-lite：如何按周聚合

Question

我已经看到可以使用多个时间单位进行聚合，例如按月而不是按周。

我已经看到在 vega 中可以自定义时间单位 https://vega.github.io/vega/docs/transforms/timeunit/#chronological-time-units

是否可以在 vega-lite 中使用它并按周聚合，并在示例中将 this aggregation 从月转换为周？

谢谢

Answer 1

您可以使用步长为 7 的 monthdate timeUnit 按周分组：

"timeUnit": {"unit": "monthdate", "step": 7}

例如：

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {"url": "data/seattle-temps.csv"},
  "mark": "line",
  "encoding": {
    "x": {"timeUnit": {"unit": "yearmonthdate", "step": 7}, "field": "date", "type": "temporal"},
    "y": {"aggregate": "mean", "field": "temp", "type": "quantitative"}
  }
}

不过请注意，这会在每个月初开始新的一周，这意味着如果您按星期几和星期几做热图，就会有间隔：

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {"url": "data/seattle-temps.csv"},
  "mark": "rect",
  "encoding": {
    "y": {"timeUnit": "day", "field": "date", "type": "ordinal"},
    "x": {"timeUnit": {"unit": "yearmonthdate", "step": 7}, "field": "date", "type": "ordinal"},
    "color": {"aggregate": "mean", "field": "temp", "type": "quantitative"}
  }
}

如果您想要更细粒度地控制周的开始位置，很遗憾，这不能表示为 timeUnit，但您可以利用 Vega-Lite 的完整 transform 语法来制作更多自定义聚合。例如，这里我们通过计算数据中的星期日来计算一年中的一周：

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {"url": "data/seattle-temps.csv"},
  "transform": [
    {"timeUnit": "yearmonthdate", "field": "date", "as": "date"},
    {
      "aggregate": [{"op": "mean", "field": "temp", "as": "temp"}],
      "groupby": ["date"]
    },
    {"calculate": "day(datum.date) == 0", "as": "sundays"},
    {
      "window": [{"op": "sum", "field": "sundays", "as": "week"}],
      "sort": "date"
    }
  ],
  "mark": "rect",
  "encoding": {
    "y": {"timeUnit": "day", "field": "date", "type": "ordinal", "title": "Day of Week"},
    "x": {"field": "week", "type": "ordinal", "title": "Week of year"},
    "color": {"aggregate": "mean", "field": "temp", "type": "quantitative"}
  }
}