First commit for violinplot

[imperorrp]

First commit for violinplot ( #14 )

plot_violin computes kde marginal densities like plot_dist does and then flips the 'x' and 'y' values to generate the characteristic violin-shape.

• Separate artists have been defined for 'left' and 'right' densities and thus can be customized aesthetically or switched on/off with relevant plot_kwargs. In addition, I've also computed them separately so that this computation can also be customized through stats_kwargs, through things like bandwidth and others.

• The line_xy visual element was modified with a new keyword arg 'negative' that makes either the sel(plot_axis='x') or sel(plot_axis='y') dataarrays negative

• New visual elements were added for line_y and scatter_y which are pretty much symmetrical to line_x and scatter_x but for the other axis.

• The 'y' aesthetic mapping applied to the credible interval, point estimate and point estimate text in plot_dist was converted to an 'x' aesthetic in case of multiple models- this needs refinement though (Maybe a mapping akin to the one in the ESS Plot PR Add ESS Plot #58 )

• The remove_axis is now applied to the 'x' axis and a modification was required in the matplotlib backend for remove_axis for this

Possible improvement: having a separate 'face' and 'edge' artist for the left and right densities, like plot_ridge. This will require creating a symmetrical visual element for fill_between_x like fill_between_y.

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📚 Documentation preview 📚: https://arviz-plots--85.org.readthedocs.build/en/85/

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 9.18367% with 178 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (f4a39af) to head (6a8be2e).

Files with missing lines	Patch %	Lines
src/arviz_plots/plots/violinplot.py	6.39%	161 Missing ⚠️
src/arviz_plots/visuals/__init__.py	23.80%	16 Missing ⚠️
src/arviz_plots/backend/matplotlib/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   84.80%   78.97%   -5.84%     
==========================================
  Files          21       22       +1     
  Lines        2336     2530     +194     
==========================================
+ Hits         1981     1998      +17     
- Misses        355      532     +177     

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[imperorrp]

Current plot outputs:

azp.plot_violin(data, var_names=["mu", "tau"])

azp.plot_violin({"centered": data, "non centered": data2}, var_names=["mu", "tau"])

azp.plot_violin(data)

[imperorrp]

Added an error check preventing more than 2 models from being passed. I've tried to subselect the two models for the left and right violin densities, but it seems like two colors are still being passed over to the backends for the line visual element by pc.map, one for each of the models if they were overlapped instead. This has to be fixed somehow.