How to Generate PNGs with Matplotlib in a Headless Environment?

Generating a PNG with matplotlib when DISPLAY is undefined

The error message indicates that matplotlib is trying to use an interactive backend, which requires a display. However, the DISPLAY environment variable is not set. Setting matplotlib's backend to Agg (non-interactive) can resolve this error.

Solution Using Agg Backend

Import matplotlib and force it to use the Agg backend before importing any other matplotlib module:

import matplotlib
# Force matplotlib to not use any Xwindows backend.
matplotlib.use('Agg')

This ensures that matplotlib uses a non-interactive backend, eliminating the dependency on a display.

matplotlib Configuration

Alternatively, set the backend in your ~/.matplotlibrc configuration file:

backend: Agg

To use the Agg backend without adding any lines of code, create or edit the configuration file (~/.matplotlibrc) with the following content:

backend: Agg

By ensuring that matplotlib uses a non-interactive backend, you can generate PNG files without requiring a display. This solution is particularly useful when running scripts on servers or in headless environments where a display is not available.

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