A scatterplot matrix

We’ve now seen how to do brushing and linking across different plots. One of the typical use cases is the scatterplot matrix. Based on what we’ve seen above, we can already create this, just by adding specifications to the concat section.

Exercise - Create a scatterplot matrix of the features Weight_in_lbs, Miles_per_Gallon and Acceleration with linking and brushing as we did above.

When doing the exercise, you’ll notice that there is a lot of repetition, as the selection, marks and encoding are repeated for each plot. For this use case, vega-lite provides the repeat keyword. It allows you to extract the variable part of the specification into a separate array. When you do this, you’ll have to put the selection, marks and encoding within a separate spec again.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "title": "Scatterplot matrix",
  "data": {
    "url": "https://raw.githubusercontent.com/vega/vega/master/docs/data/cars.json"
  },
  "repeat": {
    "column": [ "Weight_in_lbs", "Miles_per_Gallon", "Acceleration" ],
    "row": [ "Weight_in_lbs", "Miles_per_Gallon", "Acceleration" ]
  },
  "spec": {
    "selection": {
      "my_selection": {"type": "interval", "empty": "none"}
    },
    "mark": "circle",
    "encoding": {
      "x": {"field": {"repeat": "column"}, "type": "quantitative"},
      "y": {"field": {"repeat": "row"}, "type": "quantitative"},
      "color": {
        "condition": {
          "selection": "my_selection",
          "value": "red"
        },
        "value": "lightgrey"
      }
    }
  }
}

This will give you this image. Try selecting a group of datapoints.