Generating a PNG with Matplotlib When DISPLAY Is Undefined

In Python, matplotlib is a popular library for creating static, animated, and interactive visualizations. However, when DISPLAY is not set in the environment, attempting to generate a PNG image using matplotlib can lead to the error message "no display name and no $DISPLAY environment variable." This error occurs because matplotlib's default backend requires an X server, which is not available in certain environments.

To address this issue, we must explicitly set matplotlib to use the Agg (Anti-Grain Geometry) backend. Agg is a non-interactive backend that does not require an X server, allowing for the creation of images in headless environments.

Here is how to implement this solution:

import matplotlib
# Force matplotlib to use the Agg backend
matplotlib.use('Agg')

Place this code at the beginning of your script, before importing any other matplotlib submodules such as pyplot. By setting the backend to Agg before importing pyplot, we ensure that matplotlib does not attempt to use an X-using backend.

Alternatively, you can set the backend permanently by modifying your .matplotlibrc configuration file. In the backend section, set the following:

backend : Agg

This global setting eliminates the need to specify the backend explicitly in your scripts.

Once the appropriate backend is configured, you can generate PNG images using matplotlib without encountering the "DISPLAY undefined" error.