How Can You Highlight Specific Data Values Using Datum in Vega-Lite?

In this guide, we're diving into the nifty world of the datum property in Vega-Lite! Ever wanted to highlight specific data values, like a threshold, in your visualizations? Look no further—you can do this easily by mapping a constant data value to an encoding channel via the datum property.

// A Single View or a Layer Specification
{
  ...,
  "mark/layer": ...,
  "encoding": {     // Encoding
    ...: {
      "datum": ..., // Value
    },
    ...
  },
  ...
}

What Problems Can Datum Solve?

1. Annotating Specific Data Points: Highlight particular data values, such as thresholds or milestones.
2. Customizing Multi-Series Charts: Differentiate between various series in a multi-series chart.
3. Enhancing Data Narratives: Add annotations to enrich the storytelling aspect of your data visualization.

Highlight a Specific Data Value

Using datum is particularly useful for annotating a specific data value. Imagine you have stock prices and you want to highlight a threshold value—e.g., $200. Here’s how you can do it with a rule mark:

// Example to highlight 200 dollars stock price
{
  "mark": {
    "type": "rule",
    "color": "red"
  },
  "encoding": {
    "x": {"datum": 200}
  }
}

Highlighting a Specific Date or Year

You can also use datum with a date-time definition. For instance, to showcase a particular year, the code might look like this:

// Example to highlight a specific year
{
  "mark": {
    "type": "rule",
    "color": "blue"
  },
  "encoding": {
    "x": {"datum": {"year": 2020}}
  }
}

Using Datum to Color Multi-series Chart

Another fantastic use of datum is in coloring a multi-series line chart. If you’re visualizing multiple data series, datum can help you assign colors to each series for better distinction.

// Example for multi-series chart
{
  "repeat": {"layer": ["A", "B", "C"]},
  "spec": {
    "mark": "line",
    "encoding": {
      "color": {"datum": "series"}
    }
  }
}

FAQ

1. What Is the Datum Property?

Q: What is the datum property in Vega-Lite?

A: The datum property allows you to set a constant data value for an encoding channel. It’s particularly useful for highlighting specific values or dates in your visualizations.

2. How Do I Highlight a Specific Value Using Datum?

Q: How do I use datum to highlight a specific value in my chart?

A: You can set a rule mark and assign the value you want to highlight to the datum property in the encoding section. Customize the mark properties like color to make the highlight stand out.

3. Can Datum Be Used for Date-Time Values?

Q: Can I use datum with date and time values?

A: Absolutely! You can define date-time objects within the datum property to highlight specific dates or years in your visualization.

There you have it! With datum, you can effortlessly annotate and highlight crucial data points, making your charts more informative and visually engaging. Now go ahead and try it out in your next Vega-Lite project!