How to Generate PNG Images with Matplotlib When DISPLAY is Undefined?

Generating a PNG with matplotlib When DISPLAY is Undefined

Problem:

When attempting to create a PNG image using matplotlib without a DISPLAY environment variable defined, you may encounter errors indicating that matplotlib cannot locate a suitable backend.

Cause:

matplotlib defaults to using an X-based backend, which requires a valid DISPLAY variable. When DISPLAY is undefined, matplotlib will raise an error.

Solution: Use the 'Agg' Backend

To resolve this issue, force matplotlib to use the "Agg" backend, which does not require a graphical display. This can be achieved by adding the following code before any other matplotlib imports:

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

Explanation:

The Agg backend is a non-interactive backend that provides a way to save figures as files without the need for a graphical display. By setting matplotlib to use this backend, you can generate PNG images even without a valid DISPLAY variable.

Alternative Solutions:

• Set the DISPLAY variable to a valid display device.
• Use a different Python module that does not rely on matplotlib for image generation.
• Set the backend in the .matplotlibrc file:

backend : Agg

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