Plotting String As X-Axis In Grafana XY Plot With Enums And SQLite

When visualizing data in Grafana, the XY plot is a powerful tool for displaying the relationship between two numerical variables. However, sometimes you may encounter scenarios where you want to use a string field as the x-axis, such as when representing categories or settings. This article explores how to plot strings on the x-axis in Grafana XY plots, using a practical example involving performance data for different device settings.

The Challenge: Using Strings as X-Axis Values

In many cases, data is not always purely numerical. Consider a situation where you're monitoring the performance of a system under different settings: 'off', 'idle', 'standby', and 'on'. You want to visualize how the performance (a numerical value) varies across these settings. The challenge lies in the fact that Grafana's XY plot primarily expects numerical data for both axes. Directly using strings on the x-axis can lead to visualization issues or errors.

Understanding the Problem Scenario

Let's delve deeper into the specific problem scenario. Imagine you have data stored in a SQLite database. This database contains performance measurements for a device, with one column representing the 'setting' (which can be 'off', 'idle', 'standby', or 'on') and another column representing the 'performance' (a real number). Your goal is to create a scatter plot in Grafana where the x-axis represents the 'setting' and the y-axis represents the 'performance'. This visualization will help you understand how the device's performance changes under different settings.

Data Source and Structure

Your data source is a SQLite database, and the table structure might look something like this:

The 'Setting' column contains string values representing the device's state, while the 'Performance' column contains numerical values indicating the device's performance level.

The Grafana Challenge

Grafana's XY plot expects numerical values for both the x and y axes. Directly using the string values from the 'Setting' column as the x-axis will not work. You need a way to translate these string values into a numerical representation that Grafana can understand.

Solution 1: Using Enums to Map Strings to Numbers

One effective solution is to use enums (enumerations) to map the string values to numerical equivalents. An enum is a data type that consists of a set of named constants. In this case, you can define an enum that maps each setting string to a unique integer.

Defining the Enum

You can define an enum like this:

• off = 0
• idle = 1
• standby = 2
• on = 3

This mapping assigns a unique numerical value to each setting string. Now, you can use these numerical values in your Grafana query and plot them on the x-axis.

Modifying the SQL Query

To use the enum in your Grafana query, you can modify your SQL query to convert the string values to their numerical equivalents. Here's how you can do it using the CASE statement in SQL:

SELECT
  CASE setting
    WHEN 'off' THEN 0
    WHEN 'idle' THEN 1
    WHEN 'standby' THEN 2
    WHEN 'on' THEN 3
  END AS setting_enum,
  performance
FROM
  your_table
ORDER BY
  setting_enum

This query creates a new column called 'setting_enum' that contains the numerical representation of the setting. You can then use this 'setting_enum' column as the x-axis in your Grafana XY plot.

Configuring the Grafana XY Plot

In Grafana, you would configure the XY plot as follows:

1. Set the data source to your SQLite database.
2. Use the SQL query above to fetch the data.
3. Set the x-axis to the 'setting_enum' column.
4. Set the y-axis to the 'performance' column.
5. Configure the series settings as needed (e.g., point size, color).

Enhancing the Visualization with Value Mappings

While the numerical x-axis now allows Grafana to plot the data, it's not very user-friendly. Users will see numbers like 0, 1, 2, and 3 instead of the actual setting names ('off', 'idle', 'standby', 'on'). To address this, Grafana provides a feature called Value Mappings. Value mappings allow you to map numerical values to human-readable strings.

To use value mappings, you would:

1. Go to the Axis tab in the XY plot panel options.
2. Find the Value Mappings section.
3. Add a new value mapping.
4. Map each numerical value (0, 1, 2, 3) to its corresponding string value ('off', 'idle', 'standby', 'on').

With value mappings in place, the x-axis will now display the setting names, making the visualization much easier to understand.

Solution 2: Transforming Data with Grafana Transformations

Another approach is to use Grafana's built-in Transformations. Transformations allow you to manipulate data within Grafana itself, without modifying the original SQL query. This can be useful if you don't have direct control over the database schema or query.

Using the "Add Field From Calculation" Transformation

One transformation that can help is the "Add field from calculation" transformation. This transformation allows you to create a new field based on a calculation applied to existing fields. In this case, you can use it to create a numerical representation of the setting string.

1. Fetch the Data: Start by fetching the data from your SQLite database using a simple query like SELECT setting, performance FROM your_table. This will give you the raw string values for the setting.
2. Add Transformation: In the Grafana panel editor, go to the "Transform" tab and add a new transformation.
3. Select Transformation: Choose the "Add field from calculation" transformation.
4. Configure Calculation:
   • Mode: Select "Binary operation".
   • Operation: Choose "Replace".
   • Field name: Enter the name of the new field (e.g., "setting_enum").
   • Input: Use expressions to map the string values to numbers. For example:
     • `if setting ==