Fix premature plt.close() resulting in no subplots saved

[tomvothecoder]

Description

Related to #987.

In #984, I refactored plotting utilities to make them more maintainable by sharing logic across plotters.

During refactoring, I added a call to plt.close() in _save_main_plot() (here). This prematurely closes the active plot figure, which results in subsequent calls to the plt library to produce blank figures -- including subplots in PDF format.

This PR now calls plt.close(fig) after all plots have been saved. It also replaces plt.close() with plt.close(fig) to ensure that the correct figure is closed.

• plt.close() without arguments closes the current figure, which may not be the one you just saved, especially if you have multiple figures open.
• plt.close(fig) ensures you are closing the specific figure you created and saved, which is safer and more robust in scripts and libraries.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[tomvothecoder]

Hey @chengzhuzhang, this PR is ready for review. Subplots are now generated in the PDF.

Please merge when ready.

Example: https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.tvo/tests/pdf_size_1/zonal_mean_2d/ERA5/ERA5-T-ANN-global.0.pdf

Full directory:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.tvo/tests/pdf_size_1/zonal_mean_2d/ERA5/

[tomvothecoder]

Fix mypy pre-commit error.

[tomvothecoder]

Example of a change to call plt.close(fig) AFTER saving all possible plots.

[tomvothecoder]

Moved these kinds of functions at the bottom because they are the last called private function, makes it easier to read file.

[tomvothecoder]

This line is what causes subsequent subplots to be missing in the PDF.

[chengzhuzhang]

I re-ran the tropical_subseasonal set, and now the pdfs look as expected.