How Can I Customize Headers in Vega-Lite Faceted Plots?

Understanding how to customize headers in Vega-Lite is crucial for creating clear and informative faceted plots. In this guide, we’ll cover what headers are, how they work by default, and how you can customize them to meet your needs.

What are Headers?

Headers provide a title and labels for faceted plots. A header's title describes the field used to facet the plot, while the header's labels describe that field's value for each subplot.

By default, Vega-Lite creates headers with default properties for the row and column channels of a faceted view. However, you can customize these headers using the header property.

Example of Default Headers

Imagine you have a dataset of cars with the fields Name, Year, and Horsepower. If you create a faceted plot by Year, Vega-Lite will automatically generate headers with default settings for each subplot.

How to Customize Headers?

You can set the header property of row or column field definitions to customize header properties. This can also be globally configured via the config object.

Example Configuration

Here’s an example of how you can customize the headers in a Vega-Lite specification.

// A Single View or a Layer Specification
{
  ...,
  "mark/layer": ...,
  "encoding": {
    "row": {
      "field": ...,
      "type": ...,
      "header": {
        "title": "Custom Title",
        "labelFont": "Arial",
        "labelFontSize": 12
      },
      ...
    },
    "x": ...,
    "y": ...,
    ...
  }
}

// A Facet Specification
{
  ...,
  "facet": {
    "row/column": {
      "field": ...,
      "type": ...,
      "header": {
        "title": "Facet Title",
        "titleFontSize": 16,
        "labelAngle": 45
      },
      ...
    },
    ...
  },
  "spec": ...
}

Customizable Header Properties

Headers can be customized using several groups of properties, such as general settings, labels, and titles.

General Settings:

• title: Sets the header title.
• Other general properties can be referenced here.

Label Settings:

• labelFont, labelFontSize, labelColor, etc.: Customize the font and appearance of labels.

Title Settings:

• titleFont, titleFontSize, titleColor, etc.: Customize the font and appearance of the title.

Example

Here’s a more concrete example:

{
  "data": { "url": "data/cars.json" },
  "facet": { "row": { "field": "Origin" } },
  "spec": {
    "mark": "point",
    "encoding": {
      "x": { "field": "Horsepower", "type": "quantitative" },
      "y": { "field": "Miles_per_Gallon", "type": "quantitative" },
      "color": { "field": "Cylinders", "type": "ordinal" }
    }
  },
  "config": {
    "header": {
      "titleFontSize": 14,
      "labelFontSize": 12,
      "labelAngle": 90
    }
  }
}

This configuration changes the font size of the titles and labels and sets the label angle to 90 degrees.

FAQ – All About Headers in Vega-Lite

1. Can I customize header properties for individual facets?

Yes, you can customize header properties at the individual facet level by setting the header property in the row or column field definitions.

2. What happens if I set header properties in both the config object and individual field definitions?

The header properties set in the individual field definitions will override the properties set in the config object.

3. Can I use CSS to style headers?

Vega-Lite supports style configurations for common properties, including for headers. Check out guide-title and guide-label for more information.

Now you're ready to make your faceted plots not only informative but also visually appealing. Happy charting with Vega-Lite!