Time is an important aspect of many datasets, and being able to discretize and visualize it properly is essential. In Vega-Lite, you can easily handle time units to make your time-based data visualizations more meaningful and engaging. Let's explore how you can use time units in different ways within Vega-Lite.

What Problems Do Time Units Solve in Data Visualization?

Breaking Down Complex Time Data

Imagine you have a dataset with timestamps ranging over several years. Directly plotting this data might become too cluttered to interpret. Using time units, you can break down this complexity into more digestible pieces, like months, days, or even hours.

Highlighting Periodic Patterns

Data often contains seasonal patterns and trends. Using time units like "month" or "day" can help highlight these periodic trends, making them easily visible in your graphs.

Syncing with Local or UTC Time

Depending on your needs, you might want to visualize data in local time or in Coordinated Universal Time (UTC). Vega-Lite allows you to choose and manage these settings effortlessly.

Main Ways to Use Time Units in Vega-Lite

In Encoding Field Definition

Time units can be applied directly to the encoding fields in your Vega-Lite specifications. For example, to plot temperature against months:

{
  "data": { "url": "data/seattle-weather.csv" },
  "mark": "line",
  "encoding": {
    "x": {
      "timeUnit": "month",
      "field": "date",
      "type": "temporal"
    },
    "y": {
      "aggregate": "mean",
      "field": "temp",
      "type": "quantitative"
    }
  }
}

In this example, the x-axis is broken down by month, making monthly trends easily discernible.

As a Standalone Transform

Time unit transformations can also be applied independently, allowing more flexibility in the manipulation of time data before plotting.

{
  "data": { "url": "data/seattle-weather.csv" },
  "transform": [
    {
      "timeUnit": "month",
      "field": "date",
      "as": "month"
    }
  ],
  "mark": "line",
  "encoding": {
    "x": {
      "field": "month",
      "type": "ordinal"
    },
    "y": {
      "aggregate": "mean",
      "field": "temp",
      "type": "quantitative"
    }
  }
}

Using this transform, the month is extracted first and then visualized.

Using UTC Time Units

For global datasets, you might need to use UTC time units to ensure consistency across different timezones.

{
  "data": { "url": "data/seattle-weather.csv" },
  "mark": "line",
  "encoding": {
    "x": {
      "timeUnit": "utcyearmonth",
      "field": "date",
      "type": "temporal"
    },
    "y": {
      "aggregate": "mean",
      "field": "temp",
      "type": "quantitative"
    }
  }
}

The use of utcyearmonth ensures that the time data is interpreted in UTC.

FAQ

1. What happens if I don't specify a time unit?

If you don't specify a time unit, Vega-Lite will plot your date-time data continuously. This might be fine for small datasets, but for larger datasets, it’s often better to use time units to improve readability.

2. Can I combine multiple time units?

Yes, you can combine multiple time units (e.g., yearmonthdate) to handle the desired granularity in your data visualization.

3. How can I use custom intervals?

You can customize intervals using the step parameter. For example, you can aggregate data in 5-minute intervals by using "step": 5 within your time unit definition.

Now that you understand the basics and benefits of using time units in Vega-Lite, you have the tools to make your time-based data visualizations more organized and insightful. Happy visualizing!