After installing matplotlib importing it won't work as PIL (dev-python/pillow) has not been installed as a dependency:

python
Python 3.9.18 (main, Oct 11 2023, 01:10:10)
[GCC 12.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.9/site-packages/matplotlib/_init_.py", line 109, in <module>
from . import _api, _version, cbook, docstring, rcsetup
File "/usr/lib/python3.9/site-packages/matplotlib/rcsetup.py", line 27, in <module>
from matplotlib.colors import Colormap, is_color_like
File "/usr/lib/python3.9/site-packages/matplotlib/colors.py", line 51, in <module>
from PIL import Image
ModuleNotFoundError: No module named 'PIL'