Acessing array in vega lite

[Juan_Grados]

I need to perform an operation in vega-lite/Kibana 6.5 similar to the next one. I need to divide y axis by "data.values[0].b". How can I perform this operation?

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A", "b": 28}, {"a": "B", "b": 55}, {"a": "C", "b": 43},
      {"a": "D", "b": 91}, {"a": "E", "b": 81}, {"a": "F", "b": 53},
      {"a": "G", "b": 19}, {"a": "H", "b": 87}, {"a": "I", "b": 52}
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {"field": "a", "type": "ordinal"},
    "y": {"field": "b", "type": "quantitative"}
  }

[abdon (Abdon Pijpelink)]

Try this:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A", "b": 28},
      {"a": "B", "b": 55},
      {"a": "C", "b": 43},
      {"a": "D", "b": 91},
      {"a": "E", "b": 81},
      {"a": "F", "b": 53},
      {"a": "G", "b": 19},
      {"a": "H", "b": 87},
      {"a": "I", "b": 52}
    ]
  },
  "transform": [
    {
      "window": [
        {"op": "first_value", "field": "b", "as": "b2"}
      ]
    },
    {"calculate": "datum.b / datum.b2", "as": "b3"}
  ],
  "mark": "bar",
  "encoding": {
    "x": {"field": "a", "type": "ordinal"},
    "y": {"field": "b3", "type": "quantitative"}
  }
}