Is matplotlib an optional requirement?

In testing a conda build of this project I am stopped by the following error:

+ em_bright_extract --help
Traceback (most recent call last):
  File "/home/duncan/opt/miniconda3/conda-bld/em-bright_1631712644951/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/bin/em_bright_extract", line 6, in <module>
    from ligo.em_bright.utils import extract
  File "/home/duncan/opt/miniconda3/conda-bld/em-bright_1631712644951/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/lib/python3.9/site-packages/ligo/em_bright/utils.py", line 26, in <module>
    import matplotlib.pyplot as plt
ModuleNotFoundError: No module named 'matplotlib'

Is matplotlib a missing runtime requirement of the project, or just an optional extra needed for this script?