vega-lite: how to aggregate by week

Question

I have seen that it's possible aggregate using several time units, in example by month, but not by week.

And I have seen that in vega it's possible to customize the time unit https://vega.github.io/vega/docs/transforms/timeunit/#chronological-time-units

Is it possible to use it in vega-lite and aggregate by week, and transform in example this aggregation from month to week?

Thank you

Answer 1

You can group by week using a monthdate timeUnit with a step size of 7:

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

For example:

{
  "$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"}
  }
}

Note, however, that this starts a new week at the beginning of each month, which means if you do a heatmap by day of week and week there are gaps:

{
  "$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"}
  }
}

If you want more fine-grained control over where weeks start, that's unfortunately not expressible as a timeUnit, but you can take advantage of Vega-Lite's full transform syntax to make more customized aggregates. For example, here we compute the week-of-year by counting Sundays in the data:

{
  "$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"}
  }
}