How to Stack Fields in Vega-Lite to Create Stunning Visuals

When you're working with data visualization, the ability to stack fields can greatly enhance your charts, making them clearer and more informative. In Vega-Lite, you have two main ways to stack fields: using the stack property in an encoding field definition, or by employing a stack transform within the transform array.

Let's dive into each method and explore various types of stacked charts to make your data come to life!

Using the stack Property in Encoding Field Definitions

The simplest way to stack fields is by adding the stack property to your encoding field definitions. Here's a quick peek at how your JSON might look:

// A Single View or a Layer Specification
{
  "mark": "bar",  // or "area" for area charts
  "encoding": {
    "x": {"field": "category", "type": "nominal"},
    "y": {"field": "value", "type": "quantitative", "stack": "zero"},
    "color": {"field": "group", "type": "nominal"}
  }
}

By adding stack: "zero", you indicate that you want your bars or areas to stack on top of each other.

Types of Stacked Charts

Stacked Bar Chart

Creating a stacked bar chart is as simple as adding a color field. Vega-Lite handles the rest by default.

{
  "mark": "bar",
  "encoding": {
    "x": {"field": "year", "type": "ordinal"},
    "y": {"field": "count", "type": "quantitative", "stack": "zero"},
    "color": {"field": "type", "type": "nominal"}
  }
}

Stacked Area Chart

Just like with bar charts, adding a color field to your area chart creates a beautiful stacked visual.

{
  "mark": "area",
  "encoding": {
    "x": {"field": "year", "type": "ordinal"},
    "y": {"field": "value", "type": "quantitative", "stack": "zero"},
    "color": {"field": "category", "type": "nominal"}
  }
}

Normalized Stacked Charts

If you want to show proportions instead of raw values, set stack to "normalize" to create percentage-based stacked charts.

{
  "mark": "bar",
  "encoding": {
    "x": {"field": "year", "type": "ordinal"},
    "y": {"field": "value", "type": "quantitative", "stack": "normalize"},
    "color": {"field": "category", "type": "nominal"}
  }
}

Streamgraph

For a streamgraph effect, set stack to "center".

{
  "mark": "area",
  "encoding": {
    "x": {"field": "year", "type": "ordinal"},
    "y": {"field": "value", "type": "quantitative", "stack": "center"},
    "color": {"field": "category", "type": "nominal"}
  }
}

Handling Layered and Diverging Stacked Charts

Layered Bar Chart

Using stack: null layers the marks on top of each other. You can experiment with opacity to create visually appealing layered charts.

{
  "mark": {"type": "bar", "opacity": 0.6},
  "encoding": {
    "x": {"field": "year", "type": "ordinal"},
    "y": {"field": "value", "type": "quantitative"},
    "color": {"field": "category", "type": "nominal"}
  }
}

Diverging Stacked Bar Chart

Vega-Lite can also handle negative values to create diverging stacked bar charts.

{
  "mark": "bar",
  "encoding": {
    "x": {"field": "category", "type": "nominal"},
    "y": {"field": "value", "type": "quantitative", "stack": "zero"},
    "color": {"field": "group", "type": "nominal"}
  }
}

Sorting and Custom Sorting

By default, stacked bars are sorted by the stack grouping fields (such as color). You can change this by mapping another field to the order channel.

For custom sorting, derive a new field using the calculate transform and sort by that instead.

Advanced Stacking with the Stack Transform

You can also use the stack transform inside a transform array to create more complex charts.

{
  "transform": [
    {
      "stack": "value",
      "groupby": ["category"],
      "offset": "zero",
      "sort": [{"field": "value", "order": "descending"}],
      "as": ["stack_start", "stack_end"]
    }
  ],
  "mark": "bar",
  "encoding": {
    "x": {"field": "year", "type": "ordinal"},
    "y": {"field": "value", "type": "quantitative"},
    "color": {"field": "category", "type": "nominal"}
  }
}

Examples of Advanced Stacked Charts

• Diverging Bar Chart with additional transforms for complex data structures.
• Mosaic Chart with dual-direction stacking combined with window transforms.

FAQ

1. What is the purpose of using the stack property in data visualization?

The stack property helps to visually aggregate data by stacking bars or areas on top of each other, making it easier to compare cumulative values across categories.

2. How do I normalize stacked charts in Vega-Lite?

Set the stack property to "normalize" in your encoding field definition to convert raw values into percentages, giving you a normalized stacked chart.

3. Can I control the order of stacked items?

Yes, you can use the order channel in your encoding definition to specify the stacking order. For more advanced sorting, use the calculate transform to derive custom sorting fields.

Ready to stack your way to better data visualizations? Join us at vizGPT and start making your Vega-Lite charts more effective today!