Matplotlib doesn't produce plots when scripts run in tmux-CLI

[anmolkabra]

When running in tmux in the Command Line Interface (X switched off), Matplotlib dumps core while generating plots, raising this error:

QXcbConnection: Could not connect to display ':0'

---

Not a big problem as testing in CLI, after detaching from tmux, is fine. The code works like this:

try:
    os.environ["DISPLAY"]
except KeyError as e:
    matplotlib.use("Agg")

which should force matplotlib to not use a GUI backend when running in CLI. os.environ["DISPLAY"] raises a KeyError in this case. However, when the scripts are run from a tmux session in the CLI, os.environ["DISPLAY"] outputs ":0" (the monitor's variable) instead of raising the KeyError.