Refactor ECDF code

[sethaxen]

Description

This PR is a major refactor of the ECDF plot code to remove dead code, reduce code duplication, use more descriptive variable/function names, and make it easier to make the changes detailed in #2309. It makes no changes to the functionality of plot_ecdf.

All utility functions have been moved to arviz.stats.ecdf_utils. It might be worth exporting ecdf_confidence_band, but this can be left for a future PR.

Example

This example shows that public functionality is unchanged.

import numpy as np
import scipy.stats

import matplotlib.pyplot as plt
import arviz as az

np.random.seed(0)

dist = scipy.stats.expon(1)
x = dist.rvs(100)

fig, axs = plt.subplots(1, 3, figsize=(15, 4))
az.plot_ecdf(x, ax=axs[0])
az.plot_ecdf(x, ax=axs[1], confidence_bands=True, cdf=dist.cdf)
az.plot_ecdf(x, ax=axs[2], confidence_bands=True, cdf=dist.cdf, pit=True, difference=True)

On main

This PR

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

---

📚 Documentation preview 📚: https://arviz--2311.org.readthedocs.build/en/2311/

[OriolAbril]

This is amazing, thanks!

After this it will probably be possible to use these functions in plot_ppc(kind="cumulative") and in plot_loo_pit instead of each having their own subset of ecdf functionality

[OriolAbril]

I didn't know the ids argument existed, I love it!

[sethaxen]

After this it will probably be possible to use these functions in plot_ppc(kind="cumulative") and in plot_loo_pit instead of each having their own subset of ecdf functionality

Yeah! I also think it would be great if the plots where one could use kind to select the representation of some distribution (e.g. plot_dist) also supported ecdf (or cumulative?) and dot options.

I've made all requested changes.

[OriolAbril]

left one last documentation related comment but looks great

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (44db4c8) 86.73% compared to head (090af26) 86.79%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
arviz/plots/ecdfplot.py	67.85%	9 Missing ⚠️
arviz/stats/ecdf_utils.py	96.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2311      +/-   ##
==========================================
+ Coverage   86.73%   86.79%   +0.05%     
==========================================
  Files         122      123       +1     
  Lines       12705    12730      +25     
==========================================
+ Hits        11020    11049      +29     
+ Misses       1685     1681       -4     

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