Pass the `backend` in `holoviews.opts.defaults`

[LecrisUT]

This is a quick issue I've found when trying to run:

holoviews.opts.defaults(
    holoviews.opts.Curve(
        fig_size=500,
        linewidth=1,
    ),
    backend="matplotlib",
)

Because the backend is not propagated when doing _expand_options you get:

WARNING:param.main: Option 'fig_size' for Curve type not valid for selected backend ('bokeh'). Option only applies to following backends: ['matplotlib']

It seems that the options are indeed not populated due to the warning (confirmed, that with this PR, the default is set appropriately)

[ahuang11]

Thank you for contributing! To make HoloViews more robust do you think you can add a test that checks if opts are propagated properly using hv.opts.defaults or alternatively checking if the warning is gone?

Maybe in https://github.com/holoviz/holoviews/blob/main/holoviews/tests/plotting/test_plotutils.py (not 100% certain).

[LecrisUT]

If I have a reference test, I could try and add that.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.86%. Comparing base (7aaeebc) to head (d287960). Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6331      +/-   ##
==========================================
+ Coverage   88.83%   88.86%   +0.03%     
==========================================
  Files         328      328              
  Lines       69805    69831      +26     
==========================================
+ Hits        62010    62055      +45     
+ Misses       7795     7776      -19     

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[hoxbro]

Thank you for the PR, this will be part of the next release 1.21.0.

[github-actions[bot]]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.