Ruth Comer

Presumably `load_raw` is returning 24 2D cubes, and `merge` then creates the new time dimension. `load` returns a pre-merged list, so the time dimension already exists, so you need `concatenate`...

The comment on that test suggests that the grid is supposed to be blue and the colours were caused by a matplotlib bug. 😕 https://github.com/SciTools/iris/blob/83309c32a6c9cfbd4603732a4075bae0f6f07d45/lib/iris/tests/test_mapping.py#L241-L244

Maybe the fact that this test result has changed is telling us we can now remove the extra handling in `iris.plot.outline` 🤔 https://github.com/SciTools/iris/blob/83309c32a6c9cfbd4603732a4075bae0f6f07d45/lib/iris/plot.py#L1276-L1280

History: #109

Dark mode is already showing in the latest docs 😕 

Relevant test output ``` [gw2] [ 4%] FAILED docs/gallery_tests/test_gallery_examples.py::test_plot_example[plot_hovmoller] [gw1] [ 8%] PASSED docs/gallery_tests/test_gallery_examples.py::test_plot_example[plot_wind_barbs] [gw11] [ 12%] PASSED docs/gallery_tests/test_gallery_examples.py::test_plot_example[plot_polynomial_fit] [gw13] [ 16%] PASSED docs/gallery_tests/test_gallery_examples.py::test_plot_example[plot_lineplot_with_legend] [gw16] [ 20%] PASSED docs/gallery_tests/test_gallery_examples.py::test_plot_example[plot_anomaly_log_colouring] [gw20]...

~Note:~ * ~[this](https://github.com/SciTools/iris/blob/4fa3f06caf2175b8654fdb4a2bcd83080b7a1418/docs/Makefile#L58-L62) will not work so should either update or remove.~ * ~[this advice](https://scitools-iris.readthedocs.io/en/stable/developers_guide/contributing_documentation_full.html#testing) may need updating (currently the Makefile in docs/src does not have gallerytest).~ Done.

~Possibly should have some version of the setUp and tearDown here.~ ~[https://github.com/SciTools/iris/blob/4fa3f06caf2175b8654fdb4a2bcd83080b7a1418/lib/iris/tests/__init__.py#L1220-L1236]~ Done

> I pulled the `check_graphic` method off the `GraphicsTest` class so we can call it as a stand-alone function. Huh. I see @wjbenfold already did that at #4759.

I had a rethink on the test strategy: previously we created each figure and called `check_graphic` on it before creating the next figure. Current code creates all the figures for...