How Do I Create a Trend Line with Loess in Vega-Lite?

Looking to create a smooth trend line that highlights the pattern in your scatterplot data? The loess transform in Vega-Lite has got you covered! This method uses locally-estimated scatterplot smoothing to produce a beautiful, easy-to-interpret trend line. Let's dive in!

What is Loess?

Loess stands for "locally estimated scatterplot smoothing," a nifty technique that applies local weighted regressions over a sliding window of nearest-neighbor points. This transform helps in visualizing the underlying trend in your data. For options more aligned with standard parametric regression, explore regression transforms.

Here's how you can specify the loess transform within a Vega-Lite chart:

{
  ...
  "transform": [
    {"loess": ...} // Loess Transform
     ...
  ],
  ...
}

Loess Transform Definition

Here's what you need to know about the properties you can use with the loess transform:

• loess: The field to analyze.
• on: The field to use as the predictive variable.
• groupby: (Optional) Fields to group by for separate trend lines.
• bandwidth: The proportional size of the sliding window (usually between 0 and 1).
• as: (Optional) The names for the output fields.

How to Use Loess Transform

Let's look at a basic example to understand how loess transform is applied.

{
  "loess": "y",
  "on": "x",
  "bandwidth": 0.5
}

In this example, we're creating a loess trend line that models the field y as a function of x with a bandwidth of 0.5. The bandwidth helps control the smoothness of the trend line.

The output data might look like this:

[
  {"x": 1, "y": 2.3},
  {"x": 2, "y": 2.9},
  {"x": 3, "y": 2.7},
  ...
]

Want to create separate trend lines for different groups in your data? Just use the groupby parameter!

Example

Here's a practical example where we use the loess transform in a chart:

FAQ

1. What is the purpose of the bandwidth parameter?

The bandwidth parameter controls the width of the sliding window used to smooth the data. A smaller bandwidth results in a more wiggly trend line, whereas a larger bandwidth produces a smoother line.

2. Can I generate multiple trend lines based on different groups in my data?

Absolutely! You can use the groupby parameter to specify fields to group by. Separate trend lines will be created for each group, providing more granular insights.

3. How is loess different from standard regression techniques?

Loess is non-parametric and applies local weighted regressions, making it more flexible in capturing the underlying pattern in your data. Standard regression might not capture these local variations as effectively.

Feel empowered? Go ahead and add some smooth trend lines to your scatterplots, making your data visualization much more insightful and engaging!