sct_segment_graymatter: workaround for careless pyplt use
sct_segment_graymatter: workaround for careless pyplt use


Former-commit-id: 7047cca982d054ff1d4a65bc18f63d7f30acf94c
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox