How Can You Customize Axes in Vega-Lite?

Axes are essential in data visualization. They help represent the data range in a clear and understandable way. In this guide, we'll dive into how you can create and customize axes in Vega-Lite to better communicate your data.

Why Are Axes Important?

Axes provide lines, ticks, and labels that help convey how a positional range translates to data values. In simpler terms, they make scales understandable. By default, Vega-Lite auto-generates axes for the x and y channels when data fields are encoded.

But, what if you want to customize these axes to better suit your needs? Let's explore how!

Default Axis Properties

By default, Vega-Lite will add axes for x and y channels. However, you can customize these properties by defining an axis object or removing it entirely by setting axis to null.

Here's a simple structure:

{
  "mark": "point",
  "encoding": {
    "x": {
      "field": "dataFieldX",
      "type": "quantitative",
      "axis": { /* Customize axis properties here */ }
    },
    "y": {
      "field": "dataFieldY",
      "type": "quantitative"
    }
  }
}

Customizing Axis Properties

General Properties

You can customize various general properties such as alignment, extent, position, and style. For instance, by setting minExtent, you can align axes between multiple plots in a multi-view display.

Example:

{
  "axis": {
    "minExtent": 30
  }
}

Domain Properties

Domain properties include the look and feel of the axis lines. You can control the color, opacity, width, and even make the lines dashed.

Example:

{
  "axis": {
    "domain": true,
    "domainColor": "red",
    "domainWidth": 2
  }
}

Label Properties

Labels on axes can also be customized. You can format, align, style, and even apply expressions to labels.

Example:

{
  "axis": {
    "labelAngle": -45,
    "labelColor": "blue",
    "labelFontSize": 12,
    "labelExpr": "datum.label.toUpperCase()"
  }
}

Tick Properties

Tick properties affect the ticks on the axes. You can customize their count, size, color, and several other properties.

Example:

{
  "axis": {
    "tickColor": "green",
    "tickWidth": 2,
    "tickSize": 10
  }
}

Title Properties

You can also adjust the axes titles by setting properties like alignment, color, font size, and positioning.

Example:

{
  "axis": {
    "title": "Custom X Axis Title",
    "titleColor": "purple",
    "titleFontSize": 14
  }
}

Grid Properties

Grid lines can also be customized. You can control their appearance via dash patterns, color, opacity, and width.

Example:

{
  "axis": {
    "grid": true,
    "gridColor": "grey",
    "gridDash": [4, 2]
  }
}

Conditional Axis Properties

Sometimes, you might want to set axis properties conditionally based on data properties.

Example:

{
  "axis": {
    "tickColor": {
      "condition": {
        "test": "datum.value > 100",
        "value": "red"
      },
      "value": "black"
    }
  }
}

Axis Configuration

Instead of setting axis properties each time, you can use config to define default settings for all axes:

{
  "config": {
    "axis": {
      "labelColor": "blue",
      "titleFontSize": 14
    }
  }
}

Note: Configuration settings follow a precedence order from general to specific.

Real Chart Example: Using Lookup to Improve Axis Labels

Sometimes your data uses numeric codes that aren't meaningful on their own. The lookup transform can map those codes to descriptive labels that appear on the axis.

In this example, the second chart enriches the data with region names. Those names are then used for the x-axis labels, which makes the chart easier to interpret.

FAQs

1. How do I remove an axis in Vega-Lite?

You can remove an axis by setting its axis property to null as shown below:

{
  "encoding": {
    "x": {
      "field": "dataFieldX",
      "type": "quantitative",
      "axis": null
    }
  }
}

2. Can I customize both x and y axes differently?

Yes! Each axis can have its own set of custom properties. For example:

{
  "encoding": {
    "x": {
      "field": "dataFieldX",
      "type": "quantitative",
      "axis": {
        "labelColor": "green"
      }
    },
    "y": {
      "field": "dataFieldY",
      "type": "quantitative",
      "axis": {
        "labelColor": "blue"
      }
    }
  }
}

3. How do I make axis labels bold and italic?

You can combine font weight and style properties like this:

{
  "axis": {
    "labelFontWeight": "bold",
    "labelFontStyle": "italic"
  }
}

That's all for customizing axes in Vega-Lite! These options should give you a good starting point to make your data visualizations clear, precise, and visually appealing.