How to Generate PNG Images with Matplotlib on Systems Without a Display?

Generating PNG with Matplotlib Without Display Defined

When working with matplotlib without a graphical user interface (GUI), you may encounter an error indicating "no display name and no $DISPLAY environment variable." This error occurs because matplotlib attempts to use an X-based backend by default.

To resolve this issue for systems with no graphical interface, follow these steps:

Step 1: Force Matplotlib to Use Agg Backend

Before importing any components from the matplotlib package, include this code to force matplotlib to use the 'Agg' backend, which is non-interactive:

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

Step 2: Handle Potential Warnings

Depending on the version of matplotlib, you may receive a warning regarding the order of backend selection. Ensure this code is executed before any other calls to matplotlib modules.

Alternative: Edit .matplotlibrc

Instead of specifying the backend in code, you can modify the matplotlib configuration file, '.matplotlibrc,' located in the user's home directory:

• Open .matplotlibrc using a text editor.
• Add the line 'backend: Agg' to the file.
• Save and close the file.

This will ensure that matplotlib always uses the 'Agg' backend regardless of code execution order.

Example

Consider the following Python code:

import matplotlib
matplotlib.use('Agg')

import networkx as nx
import matplotlib.pyplot as plt

G = nx.Graph()
G.add_node(1)
G.add_nodes_from([2, 3, 4, 5, 6, 7, 8, 9, 10])

nx.draw(G)
plt.savefig("/var/www/node.png")

By adding 'matplotlib.use('Agg')' before the networkx and matplotlib imports, the error should be resolved, allowing you to generate a PNG image without a graphical interface.

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