Removed pyplot import from gwpy.cli.cliproduct
This PR removes a module-level import of pyplot
from gwpy.cli.cliproduct
which causes the backend to start up at import time, which breaks when you don't have Qt installed properly (or similar backend issues). Since Plot.show
handles this gracefully already, we should just use that.
This likely conflicts with #861, so I can handle things if we merge that one first.
cc: @areeda.