Fixing error 'Matplotlib is currently using agg'^edit

Have you ever built graphs in matplotlib or any other GUI running in docker? You will have to overcome some obstacles

1. missing X11 server inside docker
2. missing X11-related libraries to able to render your GUI elements

For example, I’m writing this simple python program

import matplotlib.pyplot as plt
import numpy as np

# evenly sampled time at 200ms intervals
t = np.arange(0., 5., 0.2)

# red dashes, blue squares and green triangles
plt.plot(t, t, 'r--', t, t**2, 'bs', t, t**3, 'g^')
plt.show()

First allow to connect to X11 server by

xhost local:root

And I want to run it in the docker container, by emitting the command

docker run -it --rm --env="DISPLAY" -v "/tmp/.X11-unix:/tmp/.X11-unix:rw" -v $PWD:/app python-container python /app/example.py

/app/examples/example.py:9: UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.
  plt.show()

To fix the error Matplotlib is currently using agg, one dependency must be installed. Below I provide a minimalist Dockerfile which includes the most important X11 library dependency libx11-6, which works fine on Ubuntu 20-22.04 and Debian 11+.

FROM python

RUN apt-get update && \
    apt-get install -y \
        python3 \
        python3-setuptools \
        libx11-6 \
        ca-certificates

RUN pip3 install numpy matplotlib

WORKDIR /app

et voilà, we have perfectly running minimalist docker container that is able to connect to a local X11 server and create GUI elements!

Note 1. To build an image run

docker build -t python-container .

Note 2. What does --env="DISPLAY" stand for? --env propagates $DISPLAY environment variable into the container.

Note 3. What -v "/tmp/.X11-unix:/tmp/.X11-unix:rw" does? It mounts X11 socket into the /tmp folder inside the container that will be used by GUI applications to render GUI elements.