Set domainMin to 6 months before max date in data - vega-lite

Question

I have the following Vega-Lite chart:
Open the Chart in the Vega Editor
Currently, I have the scale set as follows:
"scale": {"domainMin": "2021-06-01"}
However, what I really want is for the domainMin to be automatically calculated to be 6 months before the latest date in the notification_date field in the data.
I've looked at aggregate and expressions, but it's not exactly clear.
How can I get the maximum value of notification_date and subtract 6 months from it, and use that in "domainMin"?
Edit: To clarify, I don't want to filter the data. I want the user to be able to zoom out or pan to see the data outside the initial 6-month window. I get exactly what I want with "scale": {"domainMin": "2021-06-01"}, but this becomes out-of-date very quickly.

Answer 1

I have tried giving params and expr to domainMin, but I was unable to use the data fields in expr through datum.
The 2nd approach I tried will work for you, in this you will need to make use of joinaggregate/calculate/filter transforms. You will manually gather the max year and max months and then use it to filter your data.
Below is the modified config or refer the editor url:
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"width": "container",
"height": "container",
"config": {
"group": {"fill": "#e5e5e5"},
"arc": {"fill": "#2b2c39"},
"area": {"fill": "#2b2c39"},
"line": {"stroke": "#2b2c39"},
"path": {"stroke": "#2b2c39"},
"rect": {"fill": "#2b2c39"},
"shape": {"stroke": "#2b2c39"},
"symbol": {"fill": "#2b2c39"},
"range": {
"category": [
"#2283a2",
"#003e6a",
"#a1ce5e",
"#FDBE13",
"#F2727E",
"#EA3F3F",
"#25A9E0",
"#F97A08",
"#41BFB8",
"#518DCA",
"#9460A8",
"#6F7D84",
"#D1DCA5"
]
}
},
"title": "South Western Sydney Cumulative and Daily COVID-19 Cases by LGA",
"data": {
"url": "https://davidwales.github.io/nsw-covid-19-data/confirmed_cases_table1_location.csv"
},
"transform": [
{
"filter": {
"and": [
{"field": "lhd_2010_name", "equal": "South Western Sydney"},
{"not": {"field": "lga_name19", "equal": "Penrith"}}
]
}
},
{"calculate": "utcyear(datum.notification_date)", "as": "yearNumber"},
{"calculate": "utcmonth(datum.notification_date)", "as": "monthNumber"},
{
"window": [
{"op": "count", "field": "notification_date", "as": "cumulative_count"}
],
"frame": [null, 0]
},
{
"joinaggregate": [
{"field": "monthNumber", "op": "max", "as": "max_month_count"},
{"field": "yearNumber", "op": "max", "as": "max_year"}
]
},
{"calculate": "abs(datum.max_month_count-6)", "as": "min_month_count"},
{
"filter": "datum.min_month_count < datum.monthNumber && datum.max_year === datum.yearNumber"
}
],
"layer": [
{
"selection": {
"date": {"type": "interval", "bind": "scales", "encodings": ["x"]}
},
"mark": {"type": "bar", "tooltip": true},
"encoding": {
"x": {
"timeUnit": "yearmonthdate",
"field": "notification_date",
"type": "temporal",
"title": "Date"
},
"color": {
"field": "lga_name19",
"type": "nominal",
"title": "LGA",
"legend": {"orient": "top", "columns": 4}
},
"y": {
"aggregate": "count",
"field": "lga_name19",
"type": "quantitative",
"title": "Cases",
"axis": {"title": "Daily Cases by SWS LGA"}
}
}
},
{
"mark": "line",
"encoding": {
"x": {
"timeUnit": "yearmonthdate",
"field": "notification_date",
"title": "Date",
"type": "temporal"
},
"y": {
"aggregate": "max",
"field": "cumulative_count",
"type": "quantitative",
"axis": {"title": "Cumulative Cases"}
}
}
}
],
"resolve": {"scale": {"y": "independent"}}
}

Answer 2

A bit simpler approach to filter approximately the last six months of data might look like this:
"transform": [
...,
{"joinaggregate": [{"op": "max", "field": "notification_date", "as": "last_date"}]},
{"filter": "datum.notification_date > datum.last_date - 6 * 30 * 24 * 60 * 60 * 1000"}
]
It makes use of the fact that dates are stored as millisecond time-stamps, and has the benefit that it will work across year boundaries.

Answer 3

This is not quite an answer to the question you asked, but if your data is reasonably up-to-date (that is, the most recent data point is close to the current date), you can do something like this:
"scale": { "domainMin": { "expr": "timeOffset('month', now(), -6)" } }