Creating Beautiful Area Charts with Vega-Lite

Are you looking to visualize changes over time using area charts? Area charts can effectively represent data by filling the area between a line and the x-axis, making them great for showing trends. Let’s dive into how to create and customize these charts using Vega-Lite.

What is an Area Chart in Vega-Lite?

In simple terms, an area chart in Vega-Lite is a way to represent multiple data elements as a single area shape. It’s mainly used to show changes over time, either with a single area or multiple stacked areas.

Basic Area Chart Formula

{
  "data": { /* your data */ },
  "mark": "area",
  "encoding": { /* your encoding */ }
}

This basic formula will set you up for creating an area chart. Let’s break it down further with specific properties and examples.

Important Properties of Area Chart

When defining an area mark, your area mark configuration might look like this:

{
  "mark": {
    "type": "area"
  },
  "encoding": { /* your encoding */ }
}

Here's what you need to know about the additional properties you can use:

• align: Alignment of the area.
• baseline: Baseline offset of the area.
• orient: Orientation of the area.
• interpolate: Line interpolation.
• tension: Tension for line interpolation.
• line: Overlays lines on top of the area.
• point: Overlays points on top of the area.

Examples

Simple Area Chart

Let’s start with a basic example of an area chart showing unemployment trends over time.

{
  "data": { 
    "values": [
      /* your data here */
    ]
  },
  "mark": "area",
  "encoding": {
    "x": { "field": "date", "type": "temporal" },
    "y": { "field": "unemployment", "type": "quantitative" }
  }
}

This chart plots unemployment over time using a filled area.

Overlaying Lines and Points

If you want to add more details, you can overlay lines and points:

{
  "mark": {
    "type": "area",
    "line": true,
    "point": true
  },
  "encoding": { /* your encoding */ }
}

Stacked Area Chart

To create a stacked area chart, simply add a color field to distinguish different areas:

{
  "mark": "area",
  "encoding": {
    "x": { "field": "date", "type": "temporal" },
    "y": { "field": "value", "type": "quantitative" },
    "color": { "field": "category", "type": "nominal" }
  }
}

Normalized Stacked Area Chart

For a normalized view to compare relative distributions:

{
  "mark": "area",
  "encoding": {
    "x": { "field": "date", "type": "temporal" },
    "y": { "field": "value", "type": "quantitative", "stack": "normalize" },
    "color": { "field": "category", "type": "nominal" }
  }
}

Streamgraph

Want a more fluid representation? Try a streamgraph by centering the stack:

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

Ranged Area

To visualize a range (e.g., confidence intervals):

{
  "mark": "area",
  "encoding": {
    "x": { "field": "date", "type": "temporal" },
    "y": { "field": "lower_bound", "type": "quantitative" },
    "y2": { "field": "upper_bound" }
  }
}

Configuration Defaults

You can set default properties for all area charts using the top-level config:

{
  "config": {
    "area": {
      /* your area configurations */
    }
  }
}

Default configurations help maintain consistency and easily adjust settings for multiple charts.

FAQs

1. What are some common uses for area charts?

Area charts are great for visualizing time series data, showing the magnitude of change, and comparing multiple categories over time.

2. How do I customize the appearance of my area chart?

You can customize the appearance by using properties like color, opacity, line, and point in the mark definition or setting defaults in the config.

3. Can I combine different types of visualizations in the same chart?

Yes, you can overlay line and point markers over an area chart for more detailed insights. You can also stack multiple areas to compare different categories.