What Are Scales in Vega-Lite and How Can They Help in Visualizing Data?

Scales in Vega-Lite are magical functions that take your raw data values (like numbers, dates, strings, etc.) and transform them into visual values (like pixels, colors, sizes). Think of them as the translators between the data language and the visual presentation. Internally, Vega-Lite uses Vega scales (opens in a new tab), which are built upon the d3-scale (opens in a new tab) library.

Why Use Scales?

Scales are essential for effective data visualization. They allow us to map different types of data (quantitative, temporal, ordinal, etc.) to visual dimensions (position, size, color, shape). Without scales, our visualizations would be either incomprehensible or non-existent.

Getting Started with Scales in Vega-Lite

Vega-Lite automatically creates scales for fields mapped to position and mark property channels. However, you can customize these scales as per your requirements.

Basic Example of a Custom Scale:

{
  ...,
  "mark/layer": ...,
  "encoding": {
    "x": {
      "field": "some_field",
      "type": "quantitative",
      "scale": {
        "type": "linear",
        ...
      },
      ...
    },
    "y": {
      "field": "another_field",
      ...
    },
  ...
  }
}

You can also set global default scale properties using the top-level config object.

Customizing Your Scale

Scale Types

The type property allows you to specify and customize the scale type.

Vega-Lite uses default scale types based on the data and encoding channels:

• X, Y: Band/Point for Nominal/Ordinal, Linear for Quantitative, Time for Temporal.
• Size, Opacity: Linear for Quantitative and Time.
• Color: Ordinal for Nominal, Linear for Quantitative and Temporal, Bin-Ordinal for Binned Quantitative.
• Shape: Ordinal for Nominal.

Example: Different Scale Types

{
  "encoding": {
    "x": {
      "field": "time",
      "type": "temporal",
      "scale": {
        "type": "time"
      }
    },
    "y": {
      "field": "value",
      "type": "quantitative",
      "scale": {
        "type": "log"
      }
    }
  }
}

Scale Domains

By default, a scale in Vega-Lite draws domain values directly from a channel's encoded field, but you can customize this.

Example: Limiting the X Domain for a Line Plot

{
  "mark": "line",
  "encoding": {
    "x": {
      "field": "value",
      "type": "quantitative",
      "scale": {
        "domain": [300, 450]
      }
    },
    "y": {
      "field": "count",
      "type": "quantitative"
    }
  }
}

Clipping Marks Outside the Range

{
  "mark": {
    "type": "line",
    "clip": true
  },
  "encoding": {
    "x": {
      "field": "value",
      "type": "quantitative",
      "scale": {
        "domain": [300, 450]
      }
    },
    "y": {
      "field": "count",
      "type": "quantitative"
    }
  }
}

Scale Ranges

The range of the scale represents the set of output visual values.

Default Ranges:

• x: [0, width]
• y: [0, height]
• color: Uses category, ordinal, heatmap, or ramp based on field type.
• size: Depends on the mark type (bars, lines, points, etc.).

Example: Custom Color Range

{
  "encoding": {
    "x": {
      "field": "value",
      "type": "quantitative"
    },
    "color": {
      "field": "category",
      "type": "ordinal",
      "scale": {
        "range": ["red", "blue", "green"]
      }
    }
  }
}

FAQs About Scales in Vega-Lite

Q1: How can I specify a logarithmic scale for my data?

A: You can specify a logarithmic scale in the scale property within the encoding channel as follows:

{
  "encoding": {
    "x": {
      "field": "value",
      "type": "quantitative",
      "scale": {
        "type": "log"
      }
    }
  }
}

Q2: Can I manually set the colors for a chart rather than using predefined color schemes?

A: Yes, you can set the range property in the scale to an array of custom colors.

{
  "encoding": {
    "color": {
      "field": "category",
      "type": "ordinal",
      "scale": {
        "range": ["#f00", "#0f0", "#00f"]
      }
    }
  }
}

Q3: What is the difference between band and point scales?

A: Band scales create uniformly spaced bands, ideal for bar charts. Point scales create uniformly spaced points without bands, perfect for scatter plots. Both use a discrete domain to map to a continuous range.

Start leveraging scales in Vega-Lite today to create more effective and visually appealing data visualizations!