Vega Lite - set parameter array max value based on data

Question

I am relatively new to Vega Lite and have a question that I'm hoping is fairly straightforward.

I have a parameter array called myExtent that I've hard coded to [0, 6]. I'd like to be able to set the upper value of the array based on the data. Here, because the 4th row has "flag" = 1, I'd like to set the upper limit to the "score" for that row, or 6. So,

{"name": "myExtent", "value": [0, (value of score for the row in the dataset where flag = 1)]}

Is something like this possible, or is there an alternative way I should be thinking about this?

`

{"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "params": [
    {"name": "myExtent", "value": [0, 6]}
  ],
  "data": {
    "values": [
      {"game": 1, "score": 2, "flag": 0},
      {"game": 2, "score": 4, "flag": 0},
      {"game": 3, "score": 5, "flag": 0},
      {"game": 4, "score": 6, "flag": 1},
      {"game": 5, "score": 9, "flag": 0}
      ]
  },
  "mark": {"type": "area"},
  "transform": [
    {
      "density": "score",     
      "extent": {"signal": "myExtent"}
    }
  ],
  "encoding": {
    "x": {"field": "value", "type": "quantitative", "scale": {"domain": [0, 10]}},
    "y": {"field": "density", "type": "quantitative"}
  }
}

`

Just to get started, I have tried something like this:

`

  "params": [
    {"name": "upperLimit", "value": 6},
    {"name": "myExtent", "value": [0, {"expr": "upperLimit"}]}
  ],

`

However, that (a) doesn't seem to work and (b) doesn't (yet) get at how to set the upperLimit parameter to the score for row 4.

Answer 1

What is the logic behind providing a dynamic extent and not letting it calculate from the whole dataset? This is quite difficult to do in VL and you probably need Vega. Having said that, if you can add a column with the max extent repeated for the whole dataset, it can be achieved. I have repurposed the flag column here to do what you want.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "params": [
    {"name": "val", "expr": "data('source_0')[0]['flag']"},
    {"name": "myExtent", "expr": "[0,val]"}
  ],
  "data": {
    "values": [
      {"game": 1, "score": 2, "flag": 6},
      {"game": 2, "score": 4, "flag": 6},
      {"game": 3, "score": 5, "flag": 6},
      {"game": 4, "score": 6, "flag": 6},
      {"game": 5, "score": 9, "flag": 6}
    ]
  },
  "mark": {"type": "area"},
  "transform": [
    {"calculate": "datum.flag==1?datum.score:0", "as": "new"},
    {"joinaggregate": [{"op": "max", "field": "new", "as": "test"}]},
    {"density": "score", "extent": {"signal": "myExtent"}}
  ],
  "encoding": {
    "x": {
      "field": "value",
      "type": "quantitative",
      "scale": {"domain": [0, 10]}
    },
    "y": {"field": "density", "type": "quantitative"}
  }
}