How Do I Use Error Bars in Vega-Lite?

Creating error bars in Vega-Lite is a breeze! Whether you're working with raw or aggregated data, error bars help you visually represent the variability in your data, making your insights clearer and more robust. This guide will walk you through everything you need to know about error bars in Vega-Lite.

What Exactly Is an Error Bar?

An error bar is a visual summary of the variability of your data. It typically shows a range from a lower to an upper bound, representing error, standard deviation, confidence intervals, or interquartile ranges. In Vega-Lite, you can create an error bar by setting the mark type to "errorbar".

Here's a basic template for creating an error bar:

{
  "data": ... ,
  "mark": "errorbar",
  "encoding": ... ,
  ...
}

Error Bar Mark Properties

You can fine-tune your error bars using several properties:

• type: Type of the error bar.
• extent: What the error bar should represent (e.g., standard error, standard deviation).
• orient: Orientation of the error bar.
• color: Color of the error bar.
• opacity: Opacity of the error bar.

You can also define properties for different parts of the error bar like rule and ticks.

Using Error Bars with Raw Data

1. Standard Error (Default)

Default error bars in Vega-Lite show standard error. Setting extent to "stderr" makes this explicit.

Example:

{
  "data": ... ,
  "mark": {"type": "errorbar", "extent": "stderr"},
  "encoding": ...
}

Here, the length of the error bars represents the standard error from the mean.

2. Standard Deviation

Set extent to "stdev" to show standard deviation.

Example:

{
  "data": ... ,
  "mark": {"type": "errorbar", "extent": "stdev"},
  "encoding": ...
}

Here, the bars represent one standard deviation from the mean.

3. Confidence Interval

Specify extent as "ci" to show confidence intervals.

Example:

{
  "data": ... ,
  "mark": {"type": "errorbar", "extent": "ci"},
  "encoding": ...
}

The bars extend from the lower to the upper confidence interval limit.

4. Interquartile Range (IQR)

Set extent to "iqr" to visualize the interquartile range.

Example:

{
  "data": ... ,
  "mark": {"type": "errorbar", "extent": "iqr"},
  "encoding": ...
}

The bars represent values from the first to the third quartile.

Using Error Bars with Aggregated Data

Pre-Aggregated from Low to High

If your data is already aggregated with low and high values, specify x and x2 (or y and y2).

Example:

{
  "data": ... ,
  "mark": "errorbar",
  "encoding": {
    "x": {"field": "low"},
    "x2": {"field": "high"},
    ...
  }
}

Aggregated with Center and Error Values

For data aggregated with center and error values, specify x, xError, and xError2 (or y, yError, and yError2).

Symmetric Example:

{
  "data": ... ,
  "mark": "errorbar",
  "encoding": {
    "x": {"field": "center"},
    "xError": {"field": "error1"},
    ...
  }
}

Asymmetric Example:

{
  "data": ... ,
  "mark": "errorbar",
  "encoding": {
    "x": {"field": "center"},
    "xError": {"field": "negError"},
    "xError2": {"field": "posError"},
    ...
  }
}

Dimensions & Orientations

1D Error Bar:

• Horizontal: Continuous field on the x-axis.

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

• Vertical: Continuous field on the y-axis.

  {
    "encoding": {"y": {"field": "value", "type": "quantitative"}}
  }

2D Error Bar:

• Horizontal: Continuous field on the x-axis, discrete field on the y-axis.

  {
    "encoding": {
      "x": {"field": "value", "type": "quantitative"},
      "y": {"field": "category", "type": "nominal"}
    }
  }

• Vertical: Continuous field on the y-axis, discrete field on the x-axis.

  {
    "encoding": {
      "y": {"field": "value", "type": "quantitative"},
      "x": {"field": "category", "type": "nominal"}
    }
  }

Customize Your Error Bars

You can further customize your error bars by setting properties like color and opacity. You can even add tooltips for more detailed information.

Example:

{
  "data": ... ,
  "mark": {
    "type": "errorbar",
    "color": "#4682b4",
    "opacity": 0.7
  },
  "encoding": ...
}

FAQ

What Are Error Bars Used For?

Error bars are used to indicate the variability of data. They can show standard errors, standard deviations, confidence intervals, or interquartile ranges, helping you understand the precision of an estimate.

How Do I Customize the Appearance of Error Bars?

You can customize error bars by setting properties like color, opacity, and by using encoding channels to specify different parts of the error bar.

Can I Use Error Bars with Aggregated Data?

Yes, you can use error bars with aggregated data. You can specify aggregated low and high values directly or use center and error values for both symmetric and asymmetric error bars.

Now that you have a comprehensive overview, go ahead and add those sleek error bars to your Vega-Lite visualizations! Happy charting!