Trying to run matplotlib code on Code Ocean, you may encounter the following error:
RuntimeError: Invalid DISPLAY variable.
matplotlib is trying to show the figure but cannot find a display. The recommended solution is to set the default backend¹ by creating a matplotlib configuration file - a file called matplotlibrc consisting of the following line:
backend: Agg.
Alternatively, you can set the backend manually at the top of your python script:
import matplotlib
matplotlib.use('Agg') # set the backend before importing pyplot
import matplotlib.pyplot as plt # etc. etc.
"What is a backend?", matplotlib.org.