Demo of output cluttering by Matplotlib plot directive

Matplotlib plot directive in some cases copies unnecessary files to the output directory, and in some cases the necessary files are not copied. This is a minimal working example for issue #24005.

Steps to reproduce:

1. Stray files in a “clean” build:

   a. Run make clean, just in case, to ensure a clean build. b. Run make singleghtml to produce a “single HTML” build. c. Inspect the output directory build/singleghtml — in addition to the necessary file index.html and the _static and _images directories (the latter contains all the necessary images used in index.html), there are also absolutely unnecessary and unreferenced SVG and PDF files directly and inside the absolutely unnecessary and unreferenced subdirectory subdir.

2. No stray files when reusing another build:

   a. Run make clean to clean the previous build results. b. Run make html to produce a regular “HTML” build in build/html. The output also has the same unnecessary and unreferenced files, as above, but this is not the point. c. Run make singleghtml to produce a “single HTML” build, which will reuse the images already produced by the previous build. d. Inspect the output directory build/singleghtml — now it looks as expected, without any unnecessary and unreferenced SVG and PDF files and subdirectories (all used images are properly inside _images).

3. Needed files are not copied when reusing another build:

   a. Comment out plot_html_show_formats = False (and plot_html_show_source_link = False) in conf.py to enable automatic links to image formats (and plotting scripts). b. Run make clean to clean the previous build results. c. Run make html to produce a regular “HTML” build in build/html. The output has all the necessary files that are linked from the HTML, as expected. d. Run make singleghtml to produce a “single HTML” build, which will reuse the images (and scripts) already produced by the previous build. e. Inspect the output directory build/singleghtml — it doesn't have any image (and script) files, resulting in broken links to “(Source code, svg, pdf)” for all the plots in the HTML file.

4. Links with wrong subdirectory paths in singleghtml build:

   a. Use the modified conf.py from the previous build. b. Run make clean to clean the previous build results. c. Run make singleghtml to produce a “single HTML” build, which should include links to scripts and image formats. d. Inspect the output directory and confirm that build/singleghtml/subdir contains {other-1,script}.{py,pdf,svg} files. e. Open the generated HTML file build/singleghtml/index.html and try using the “(Source code, svg, pdf)” links in the “Other” section. None of them works because they all refer to ../subdir/<file> (resolving to build/subdir/<file>) instead of subdir/<file> (correct path relative to index.html, resolving to build/singleghtml/subdir/<file>).