How do I do conditional coloring based on a value in the Vega-Lite API - vega-lite

Question

I am trying to conditionally color the text of a heat map in the same style black and white text on this page: Condition
I am looking specifically at the conditional color encoding:
"encoding": {
"text": {"field": "num_cars", "type": "quantitative"},
"color": {
"condition": {"test": "datum['num_cars'] < 40", "value": "black"},
"value": "white"
}
}
I can't seem to get something similar to work in Vega-Lite. My latest version looks like this:
vl.data(weatherData)
.transform(
vl.calculate("monthAbbrevFormat(month(datum.date))").as("month"),
vl.calculate("date(datum.date)").as("day"),
vl.aggregate([{op:"average",
field:"temp_max",
as:"avg_temp"
}]).groupby(["month","day"])
)
.encode(
vl.y().fieldO("month").sort(["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]),
vl.x().fieldO("day")
)
.layer(
vl.markRect({tooltip: true, clip: true})
.encode(
vl.color().average("avg_temp").scale({scheme:"redyellowblue", reverse:true})
),
vl.markText({tooltip: true, clip: true})
.encode(
vl.text().average("avg_temp").format(".1f"),
vl.color().condition({test:"datum['avg_temp'] > 26",value:"white"}).value("black")
)
)
.width(1000)
.height(400)
.render()
When I convert that to JSON, I get
"color":{
"condition":{"test":"datum['avg_temp'] > 26", "value":"white"},
"value":"black"
}
which looks the same to my eyes. However, the text resolutely stays black.
I've put the output JSON into the Vega editor, and it also doesn't work there, so the problem isn't limited to my problems with the JavaScript API. It would great if someone could point out where my logic is failing (and also fill me in on the correct syntax in the API as the documentation is sadly lacking in examples).

Answer 1

I figured out my problem. In an earlier version I was computing the average in the encoding. I switched to pre-computing it in the transform, and didn't update the encoding for the text. So it was still
.encode(
vl.text().average("avg_temp").format(".1f"),
vl.color().condition({test:"datum['avg_temp'] > 26",value:"white"}).value("black")
)
The average wasn't changing the values, but it meant that dataum['avg_temp'] wasn't available.
If there is a way to compute the average as part of the test, that would make this a lot cleaner, but I couldn't find a way to do that.