'Vega-Lite Calculated Scale domainMax

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"}}
    }
  }
}
vega-liteyaxis


Solution 1:[1]

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.

Solution 2:[2]

  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?

  1. 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.)

  2. 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.

  3. 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.

  4. 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.)

Sources

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Source: Stack Overflow

Solution Source
Solution 1
Solution 2 az5112

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