Darkdetect appears to be a hard dependency?

[nvaytet]

In the pyproject.toml, darkdetect is listed as an optional dependency (comes with the [charts] specifier).
However, from the integration tests log that tries to import easydiffraction, it would appear that it isn't a soft dependency?

ImportError while importing test module '/home/gitlab-runner/builds/sr_srNBdt/0/dmsc-nightly/dmsc-nightly/tests/scipp-analysis/dream/cif_file_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/scipp-analysis/dream/cif_file_test.py:6: in
import easydiffraction as ed
.tox/scipp-analysis-dream/lib/python3.12/site-packages/easydiffraction/__init__.py:12: in
from easydiffraction.project import Project
.tox/scipp-analysis-dream/lib/python3.12/site-packages/easydiffraction/project.py:14: in
from easydiffraction.plotting.plotting import Plotter
.tox/scipp-analysis-dream/lib/python3.12/site-packages/easydiffraction/plotting/plotting.py:10: in
from easydiffraction.plotting.plotters.plotter_plotly import PlotlyPlotter
.tox/scipp-analysis-dream/lib/python3.12/site-packages/easydiffraction/plotting/plotters/plotter_plotly.py:4: in
import darkdetect
E ModuleNotFoundError: No module named 'darkdetect'