Bug report

• The environment variable MPLBACKEND=webagg is overwritten by 'agg' when $DISPLAY is not set on Linux.
• The behavior of backend:webagg written in matplotlibrc was as same as MPLBACKEND=webagg (I tried).
• I want to avoid hard-coding the backend to .py file, because I use the same code on other environment.

Code for reproduction

# matplotlibrc
webagg.open_in_browser : False
webagg.address : 0.0.0.0

# test.py
import matplotlib.pyplot as plt
print(plt.get_backend())

Actual outcome

When I type the shell command MPLBACKEND=WebAgg DISPLAY= python test.py,

agg

the backend is overwritten by agg.

When I type the shell command MPLBACKEND=WebAgg DISPLAY=:0 python test.py (although X11 is not installed),

WebAgg

I can use webagg.

Expected outcome

When I type the shell command MPLBACKEND=WebAgg DISPLAY= python test.py,

WebAgg

• I want to use webagg even when X11 is not installed.
• I want to use the environment variable MPLBACKEND to avoid hard-coding the backend.

Question

Does matplotlib overwrite the backend by 'agg', when $DISPLAY is empty?
I understand that the fallback behavior is useful.
However, is there any reason that webagg must be fallbacked to agg?

Or, is there a correct way (not setting the $DISPLAY) to avoid the fallback behavior?

Matplotlib version

• Operating system: Debian 10
• Matplotlib version: 3.1.0
• Python version: 3.7.3 (anaconda)