Number of classes for discrete color classes (for quantile or quantize scheme)

Given a simple bar chart with embedded data, for which I like to color encode the values using a quantize type scale (VL-docs):

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "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": "nominal", "axis": {"labelAngle": 0}},
    "y": {"field": "b", "type": "quantitative"},
    "color": {
      "field": "b",
      "type": "quantitative",
      "scale": {"type": "quantize", "scheme": "purples"}
    }
  }
}

Open the Vega-Lite Chart in the Vega Editor

I observe the default uses 5 classes for scale type quantize, but how to change the number of classes? Let’s say I want to use the color scheme purples to use 3 classes.

At the color scale page, I observe there is a discrete color schemes for purples:

But how to use these? Defining purples-3 for scheme does not work.

In Vega it would be possible to define a count as part of the scheme within range:

  {
    "name": "color",
    "type": "quantize",
    "range": {"scheme": "purples", "count": 3},
    "domain": {"data": "values", "field": "b"}
  }

Open the Vega Chart in the Vega Editor

As you can see, this successfully use 3 classes for the quantize scale. But how to approach this in Vega-Lite?

Maybe it is a regression and the style of purples-3 worked before? Since I see in this obs-notebook in the selection list of Sequential Multi-Hue Schemes it was defined as such that the suffix indicates the desired number of colors.

In this obs-notebook I observe they use range: d3.schemeBlues[9], but "scheme": "purples[3]" won’t work either.

[mattijn]

Thanks! That is what I need indeed! I was close, but still too far…

[PBI-David]

You need a scheme parameter object . I think this is what you want.

Editor

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "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": "nominal", "axis": {"labelAngle": 0}},
    "y": {"field": "b", "type": "quantitative"},
    "color": {
      "field": "b",
      "type": "quantitative",
      "scale": {"type": "quantize",  "scheme": {"name": "purples", "count": 3}}
    }
  }
}