Matplotlib is at least a test requirement

See: https://gitlab.idiap.ch/bob/bob.nightlies/-/jobs/130020

+ plot_froc.py --help
Traceback (most recent call last):
  File "/local/builds/bob/bob.nightlies/miniconda/conda-bld/bob.ip.facedetect_1523356382105/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/bin/plot_froc.py", line 7, in <module>
    from bob.ip.facedetect.script.plot_froc import main
  File "/local/builds/bob/bob.nightlies/miniconda/conda-bld/bob.ip.facedetect_1523356382105/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/lib/python2.7/site-packages/bob/ip/facedetect/script/plot_froc.py", line 25, in <module>
    import matplotlib; matplotlib.use('pdf') #avoids TkInter threaded start
ImportError: No module named matplotlib