vega-lite: how to aggregate by week - vega-lite

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"}
}
}