Show negative values in Deneb column chart in red - powerbi

Question

I've been playing around with the Deneb visual in PowerBI Desktop and (amongst many other things) have been trying to create a simple column chart that shows negative values in red and positive values in green, however can't for the life of me seem to get it working - I believe the condition/test in my script is correct, but it refuses to 'fire' when it's 'true'
I've read through the condition page of the Vega-Lite documentation https://vega.github.io/vega-lite/docs/condition.html and have a condition section within the encoding/color
I've added Month End and MonthYear columns from my Calendar table and an EBITDA measure from a fact table to the Deneb visual
Month End
MonthYear
EBITDA
31/7/2021
"Jul-21"
8277.56
31/8/2021
"Aug-21"
-15123.66
30/9/2021
"Sep-21"
9502.11
31/10/2021
"Oct-21"
13090.99
{
"data": {"name": "dataset"},
"mark": "bar",
"encoding": {
"x": {
"field": "MonthYear",
"sort": {"field": "Month End"}
},
"y": {
"field": "EBITDA",
"aggregate": "sum"
},
"color": {
"condition": {
"test": "datum['EBITDA']<0",
"value": "red"
},
"value": "green"
}
}
}
If I adjust the condition to be "test": "1==1" then the 'true' path works, so I assume I've got something wrong with my test line, though this seems to be correct per a lot of blogs, stackoverflow questions etc.
I've also tried using a "tranform:" channel to create a new Neg field in the Deneb dataset and referring to that field in my test, but it still won't adjust the colour.

Answer 1

It doesn’t like your aggregation. It looks like the data you are sending in is already aggregated by Power BI. If so, this will work:
"y": {
"field": "b",
"type": "quantitative"
},
View sample in the Vega Editor
If your data isn’t aggregated, add an aggregate transform like this:
"transform": [
{"aggregate": [{
"op": "sum",
"field": "b",
"as": "bsum"
}],
"groupby": ["a"]}
],
"mark": "bar",
"encoding": {
"x": {
"field": "a",
"sort": {"field": "a"}
},
"y": {
"field": "bsum",
"type": "quantitative"
},
"color": {
"condition": {
"test": "datum['bsum']<0",
"value": "red"
},
"value": "green"
}
}
}
Open the Chart in the Vega Editor