Counting negative and positive values of the same field with Vega-Lite

Question

I have a table with 2 dates and I would like to create a 2 sliced pie chart with 'delayed' and 'on-time' slices ('delayed' if Date2>Date1, 'on-time' if Date2<=Date1).

What strategy would you recommend to a Vega-Lite beginner?

My ideas:

1. calculate the difference in a new field. It is then unclear to me how I should express my intention in the encoding block.

{"calculate": "datum.Date1 - datum.Date2", "as":"DateDifference"}

2. bin the data -> how?

Any thoughts are highly appreciated!

Answer 1

You can do this by using a ternary operator in the calculate transform:

{
  "calculate": "toDate(datum.Date1) < toDate(datum.Date2) ? 'delayed' : 'on time'",
  "as": "diff"
}

Here's an example in a pie chart (view in editor):

{
  "data": {
    "values": [
      {"Date1": "2021-02-01T06:00:00", "Date2": "2021-02-01T05:50:00"},
      {"Date1": "2021-02-01T07:00:00", "Date2": "2021-02-01T06:49:00"},
      {"Date1": "2021-02-01T08:00:00", "Date2": "2021-02-01T08:15:00"},
      {"Date1": "2021-02-01T09:00:00", "Date2": "2021-02-01T09:05:00"},
      {"Date1": "2021-02-01T19:00:00", "Date2": "2021-02-01T09:59:00"}
    ]
  },
  "transform": [
    {
      "calculate": "toDate(datum.Date1) < toDate(datum.Date2) ? 'delayed' : 'on time'",
      "as": "diff"
    }
  ],
  "mark": "arc",
  "encoding": {
    "theta": {"field": "diff", "aggregate": "count", "type": "quantitative"},
    "color": {"field": "diff", "type": "nominal"}
  },
  "$schema": "https://vega.github.io/schema/vega-lite/v4.8.1.json"
}