Add sublabel_index and sublabel_skip option for matplotlib backend

[LecrisUT]

This allows to alter the index count of the sublabels, e.g. I have the following plot:

But the top element (a)) is not an actual subplot, just a colorbar for all the other. But exposing this option allows to basically ignore the first subplot. When numbering. In another situation I have an opposite problem, where I created a group of plots, but then I attach manually another subplot within LaTeX, then I want the numbering to skip a few labels.

[hoxbro]

Do you have a MRE I can play around with? I'm not entirely sure if this is the best way to expose this customization.

[LecrisUT]

import holoviews

holoviews.extension("matplotlib")

# For simplicity, let's just copy the plots
curve = holoviews.Curve([0, 1, 2])
layout = holoviews.Layout()
layout += curve
layout += curve
layout += curve
layout += curve
layout.cols(2)
layout.opts(
    sublabel_format=r"{alpha})",
    sublabel_index_offset=-1,
)

[LecrisUT]

I'm afraid sublabel_skip only addresses half the problem. The other half is wanting to offset the labeling, e.g. start from c) instead of a), in the example, making sublabel_index_offset=2

[hoxbro]

I pushed some changes, changing from sublabel_index_offset to sublabel_skip so you can skip multiple elements.

import holoviews as hv

hv.extension("matplotlib")

c = lambda: hv.Curve([0, 1, 2])
layout = (c() + c() + c() + c()).cols(2)
layout.opts(sublabel_skip=[1, 3])

I will look into the other part of your requests later.

[codecov]

Codecov Report

Attention: Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.50%. Comparing base (e11c52c) to head (684b823).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/plotting/mpl/plot.py	68.75%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6375    +/-   ##
========================================
  Coverage   88.49%   88.50%            
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  Files         323      323            
  Lines       68184    68311   +127     
========================================
+ Hits        60341    60458   +117     
- Misses       7843     7853    +10     

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[LecrisUT]

This interface works for my needs 👍