Conditional transforms in Vega

In the Vega documentation for transforms, this is given as an example:

{
  "data": [
    {
      "name": "table",
      "transform": [
        { "type": "filter", "expr": "datum.value > 5" },
        { "type": "stack", "field": "value", "groupby": ["category"] }
      ]
    }
  ]
}

Let two boolean signals be called is_filter and is_stack. Is it possible to conditionally execute transforms based on these signals? Something like this (which doesn't actually work):

{
  "data": [
    {
      "name": "table",
      "transform": [
        { "type": "filter", "expr": "datum.value > 5", "run_if": "is_filter"},
        { "type": "stack", "field": "value", "groupby": ["category"], "run_if": "is_stack"}
      ]
    }
  ]
}

It would be good to avoid creating a dataset for each combination of transforms and using conditional logic in the marks.

Include all the logic in the expression. Changing the is_filter signal will change the filter here:

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "A basic bar chart example, with value labels shown upon pointer hover.",
  "width": 400,
  "height": 200,
  "padding": 5,
  "data": [
    {
      "name": "table",
      "values": [
        {"category": "A", "amount": 28},
        {"category": "B", "amount": 55},
        {"category": "C", "amount": 43},
        {"category": "D", "amount": 91},
        {"category": "E", "amount": 81},
        {"category": "F", "amount": 53},
        {"category": "G", "amount": 19},
        {"category": "H", "amount": 87}
      ],
      "transform": [
        {"type": "filter", "expr": "is_filter?datum.amount > 40:datum.amount < 40"}
      ]
    }
  ],
  "signals": [
    {
      "name": "tooltip",
      "value": {},
      "on": [
        {"events": "rect:pointerover", "update": "datum"},
        {"events": "rect:pointerout", "update": "{}"}
      ]
    },
    {"name": "is_filter", "value": true}
  ],
  "scales": [
    {
      "name": "xscale",
      "type": "band",
      "domain": {"data": "table", "field": "category"},
      "range": "width",
      "padding": 0.05,
      "round": true
    },
    {
      "name": "yscale",
      "domain": {"data": "table", "field": "amount"},
      "nice": true,
      "range": "height"
    }
  ],
  "axes": [
    {"orient": "bottom", "scale": "xscale"},
    {"orient": "left", "scale": "yscale"}
  ],
  "marks": [
    {
      "type": "rect",
      "from": {"data": "table"},
      "encode": {
        "enter": {
          "x": {"scale": "xscale", "field": "category"},
          "width": {"scale": "xscale", "band": 1},
          "y": {"scale": "yscale", "field": "amount"},
          "y2": {"scale": "yscale", "value": 0}
        },
        "update": {"fill": {"value": "steelblue"}},
        "hover": {"fill": {"value": "red"}}
      }
    },
    {
      "type": "text",
      "encode": {
        "enter": {
          "align": {"value": "center"},
          "baseline": {"value": "bottom"},
          "fill": {"value": "#333"}
        },
        "update": {
          "x": {"scale": "xscale", "signal": "tooltip.category", "band": 0.5},
          "y": {"scale": "yscale", "signal": "tooltip.amount", "offset": -2},
          "text": {"signal": "tooltip.amount"},
          "fillOpacity": [
            {"test": "datum === tooltip", "value": 0},
            {"value": 1}
          ]
        }
      }
    }
  ]
}