Support using param / expr with `field` keys to control which data column is used for an encoding

I suspect there might be an open issue for this somewhere but I could not find it when searching. The new parameter feature looks very cool, but I can’t find and example of param / expr used together with the field key and when I try it in the vega editor, it says it only supports strings as values for this field. Would it be possible to add this functionality so that a field value could be set dynamically, e.g. to change which column is plotted on the x and y axis via a dropdown? Or am I not understanding how parameters are supposed to work?

A workaround for this is mention in https://github.com/vega/vega-lite/issues/7264 by first folding and then filtering on a param / expr, but it would be need if this feature was directly supported. To be clear a use case would be to create something like this (minus the cluster count calculations):

[jwoLondon]

I can confirm this works with the existing Vega-Lite implementation, although I am not sure if it is possible to customise the axis title to be data dependent. It would be useful if axis title could accept an expression in addition to a text literal.

Vega-Lite editor

{
  "data": {
    "url": "data/weather.csv"
  },
    "params": [
    {
      "name": "measure",
      "value": "temp_max",
      "bind": {
        "input": "select",
        "options": ["temp_max","precipitation","wind"]
      }
    }
  ],
  "transform": [
    {
      "calculate": "datum[measure]",
      "as": "y"
    }
  ],
  
    "mark": "line",
    "encoding": {
      "x": {"field": "date", "timeUnit": "month"},
      "y": {
        "field": "y",
        "aggregate": "mean"
      },
      "color": {"field": "location"}
    }
}

[Sebastian2023]

@jwoLondon @joelostblom Is it in Vega lite possible to select the visualized value depending on a nested field? datum[measure] from the example does not seem to be able to handle values such as Motor.Cylinder.

"data": { "values": [ { "Name": "chevrolet chevelle malibu", "Miles_per_Gallon": 18, "Motor": {"Cylinders": 8, "Horsepower": 130}, "Displacement": 307, ... "transform": [{"calculate": "datum[measure]", "as": "y"}],

Vega lite editor