How Can You Layer Views in Vega-Lite?

Have you ever needed to stack one chart on top of another to convey more insights? With Vega-Lite, you can achieve this using the layer operator. This amazing feature allows you to combine multiple charts seamlessly. Let's dive in and learn how to do it!

What is Layering?

Layering means superimposing various charts on top of each other. It can provide a richer perspective on your data by combining different visual elements. This is part of Vega-Lite's view composition operators, helping you create complex visualizations with ease.

How to Use the Layer Operator

You can define a layered chart by placing multiple chart specifications in an array under the layer property. Here's a sneak peek at the structure:

{
  "layer": [
    // Your individual or layered view specifications go here
  ]
}

In addition to common view properties, a layer specification has unique properties:

Creating a Layered Chart: An Example

Step 1: Define Individual Layers

Let's start by creating the first chart, a line chart that shows the stock price of different stocks over time.

{
  "data": {"url": "path/to/your/data"},
  "mark": "line",
  "encoding": {
    "x": {"field": "date", "type": "temporal"},
    "y": {"field": "price", "type": "quantitative"},
    "color": {"field": "stock", "type": "nominal"}
  }
}

And here's how it looks:

Step 2: Add a Second Layer

Next, let's create a rule mark indicating the mean price of individual stocks.

{
  "mark": "rule",
  "encoding": {
    "y": {"aggregate": "mean", "field": "price", "type": "quantitative"},
    "color": {"field": "stock", "type": "nominal"}
  }
}

Step 3: Combine Layers

Now, to layer these two charts on top of each other, put both specifications into the layer array, remembering to place data at the top level since both layers use the same dataset.

{
  "data": {"url": "path/to/your/data"},
  "layer": [
    {
      "mark": "line",
      "encoding": {
        "x": {"field": "date", "type": "temporal"},
        "y": {"field": "price", "type": "quantitative"},
        "color": {"field": "stock", "type": "nominal"}
      }
    },
    {
      "mark": "rule",
      "encoding": {
        "y": {"aggregate": "mean", "field": "price", "type": "quantitative"},
        "color": {"field": "stock", "type": "nominal"}
      }
    }
  ]
}

Managing Combined Scales and Guides

When different scales are present in different layers, Vega-Lite unions these scales to create a shared scale for all layers. By default, it uses a common y-axis and color legend. However, you can customize this behavior using the resolve property.

For example, to make the y-scales of different layers independent:

{
  "resolve": {"scale": {"y": "independent"}}
}

FAQ

Q1: Can I use multiple data sources in a layered chart?

A1: Yes, but each layer must specify its own data source if they are different.

Q2: What types of charts can I layer?

A2: You can layer almost any single views including bars, lines, and rules. However, complex compositions like side-by-side views cannot be directly layered.

Q3: How do I resolve conflicts in scales and legends?

A3: Use the resolve property to set independent or shared scales and legends, giving you control over how visual elements are combined.