Figure.subplot: Fix strange positioning issues after exiting subplot

[seisman]

Description of proposed changes

Need to use separate sessions for subplot begin and subplot end. Otherwise, subplot end will use the "last session" (i.e., the lib variable created by other calls), which may cause strange behaviors like #2426.

After this PR, the PyGMT example in #2426 works as expected.

TODOs:

• Add a unit test for it
• Use position="JBC+w10c/0.25c+h" in Add a gallery example showing how to use advanced grdgradient via the "azimuth" & "normalize" parameters #2354 gallery example
• Check if any tests are affected (NO)
• Check if any other examples need revision (NO)

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
added	pygmt/tests/baseline/test_subplot_outside_plotting_positioning.png

Image diff(s)

Added images

• pygmt/tests/baseline/test_subplot_outside_plotting_positioning.png

Modified images

Path	Old	New

Report last updated at commit dc0fec4

[weiji14]

Does the plot need updating? The unit test is failing at https://github.com/GenericMappingTools/pygmt/actions/runs/4427893754/jobs/7766176078#step:11:704:

__________________ test_subplot_outside_plotting_positioning ___________________
Error: Image files did not match.
  RMS Value: 3.6267715100983793
  Expected:  
    /home/runner/work/pygmt/pygmt/tmp-test-dir-with-unique-name/results/pygmt.tests.test_subplot.test_subplot_outside_plotting_positioning/baseline.png
  Actual:    
    /home/runner/work/pygmt/pygmt/tmp-test-dir-with-unique-name/results/pygmt.tests.test_subplot.test_subplot_outside_plotting_positioning/result.png
  Difference:
    /home/runner/work/pygmt/pygmt/tmp-test-dir-with-unique-name/results/pygmt.tests.test_subplot.test_subplot_outside_plotting_positioning/result-failed-diff.png
  Tolerance: 
    2

[seisman]

Minor differences are likely due to different gs versions, but I'm using gs 9.54.0 from conda-forge on macOS and the test passes for me locally:

[weiji14]

Are you using a dev version of GMT? Can you show your pygmt.show_versions()? This is the one from CI at https://github.com/GenericMappingTools/pygmt/actions/runs/4427893754/jobs/7766177139#step:11:16 which is also using ghostscript 9.54.0:

PyGMT information:
  version: v0.8.1.dev106+g52f04401
System information:
  python: 3.11.0 | packaged by conda-forge | (main, Jan 15 2023, 05:44:48) [Clang 14.0.6 ]
  executable: /Users/runner/miniconda3/envs/pygmt/bin/python
  machine: macOS-12.6.3-x86_64-i386-64bit
Dependency information:
  numpy: 1.24.2
  pandas: 1.5.3
  xarray: 2023.2.0
  netCDF4: 1.6.3
  packaging: 23.0
  contextily: 1.3.0
  geopandas: 0.12.2
  ghostscript: 9.54.0
GMT library information:
  binary version: 6.4.0
  cores: 3
  grid layout: rows
  image layout: 
  library path: /Users/runner/miniconda3/envs/pygmt/lib/libgmt.dylib
  padding: 2
  plugin dir: /Users/runner/miniconda3/envs/pygmt/lib/gmt/plugins
  share dir: /Users/runner/miniconda3/envs/pygmt/share/gmt
  version: 6.4.0

[seisman]

Are you using a dev version of GMT?

aha, you're right. I'm using GMT master.

[weiji14]

Great, now the GMT Dev Tests are failing on the main branch at https://github.com/GenericMappingTools/pygmt/actions/runs/4443039566/jobs/7799909484#step:17:718 🙃 Should we set an @pytest.mark.xfail?

[seisman]

What about increase the RMS diff threshold to 4.0 @pytest.mark.mpl_image_compare(tolerance=4.0)?