Vega-Lite Calculated Scale domainMax - vega-lite

Question

I'm trying to calculate a value for domainMax on the Y-axis scale. I tried the following example where I want the Y-axis domainMax to be one greater than the maximum value in the dataset field named "value". The example produces the error 'Unrecognized signal name: "domMax"'. How can I get it to work?
{
"data": {
"values": [
{"date": "2021-03-01T00:00:00", "value": 1},
{"date": "2021-04-01T00:00:00", "value": 3},
{"date": "2021-05-01T00:00:00", "value": 2}
]
},
"transform": [
{ "calculate": "max(datum.value)+1","as": "domMax"}
],
"mark": "line",
"encoding": {
"x": {
"field": "date",
"type": "temporal"
},
"y": {"field": "value", "type": "quantitative",
"scale": {"domainMax": {"expr": "domMax"}}
}
}
}

Answer 1

This transform
"transform": [
{ "calculate": "max(datum.value)+1","as": "domMax"}
]
adds a new column to your data set - it does not create a new signal. You can check that in the editor. Go to the DataViewer tab and select data_0 from the drop down. Can you see the new domMax column?
Signals are a different thing entirely - have a look here in the documentation. Note that the link points to Vega, not Vega-Lite. (Vega-Lite specifications are compiled to Vega.)
Vega-Lite does not let you declare signals; you declare parameters instead. Here is another example using the domMax parameter. Vega-Lite parameters are translated to Vega signals.
It looks like you are trying to derive the value of your parameter/signal from the data. I am not sure you can do that in Vega-Lite.
On the other hand it's very easy in Vega. For example you could use the extent transform:
https://vega.github.io/vega/docs/transforms/extent/
Side comment - while Vega specifications are more verbose you can sometimes find their primitives simpler and a good way to understand how the visualisation works. (You can see compiled Vega in the editor.)

Answer 2

I tried to get a custom domain based on the data but hit the same limitations as you did.
In my case, I update the data from the outside a bit like the streaming example. I compute the domain from the outside and modify them in the visualization with params. This is quite easy as vega-lite params are exposed as vega signals.
This is the gist of the layout:
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"params": [
{
"name": "lowBound",
"value": -10
},
{
"name": "highBound",
"value": 100
}
],
../..
"vconcat": [
{
"name": "detailed",
../..
"layer": [
{
../..
"mark": "line",
"encoding": {
"y": {
"field": "value",
"title": "Temperature",
"type": "quantitative",
"scale": {
"domainMin": {
"expr": "lowBound"
},
"domainMax": {
"expr": "highBound"
}
}
},
...
The lowBound and highBound are dynamically changed through vega signals. I change them with the regular JS API.

Answer 3

You can add a param to pan and zoom in case your hard coded values are less than ideal.
"params": [{"name": "grid", "select": "interval", "bind": "scales"}],
Open the Chart in the Vega Editor