I'm confused as to what issue this is solving; could you add a simple example to the test suite that raises the error?

https://matplotlib.org/devdocs/gallery/specialty_plots/sankey_basics.html#sphx-glr-gallery-specialty-plots-sankey-basics-py and https://matplotlib.org/devdocs/gallery/specialty_plots/sankey_basics.html#sphx-glr-gallery-specialty-plots-sankey-basics-py are currently completely misrendered; this PR fixes that.
As I already explained elsewhere, the sankey module is nearly entirely untested, and while it would be nice to improve the situation, that's not the point of this PR.

The test suite can't tell if the docs are being rendered correctly; correct me if I'm wrong, but adding a test that exercises the ValueError here doesn't seem like a huge job to me, and improving the test coverage bit by bit is a good thing.

But the ValueError is not actually the relevant point; what matters is the fact that if labels is a list of n strings, then the previous version of the code would create a list containing n copies of labels ([labels] * n), whereas the current version doesn't.
IOW, testing for the ValueError doesn't actually check the problem.

An image test would be better, but if I was to add one I'd rather design a proper test that actually ensures as close to 100% coverage as possible on a single image test (which I don't have right now), rather than repeatedly trying to cover incrementally more parts of the module.

Fair enough re. an image test. Would it be possible to create a diagram, then extract the resulting labels to check they're correct? Happy to look into this myself if you don't have time to add a test, but I think one should be added if possible.

Actually you can probably do this by saving to svg (with rcParams["svg.fonttype"] = "none" to avoid converting glyphs to paths) (or pdf) and then looking for the substring in the result :p

I've added a two line test that fails before this fix... I think the general argument of "it's not tested so there's no point adding tests" is a bit silly, and when we fix something as a rule there should always be a test added to make sure it doesn't break in the future.

Thanks!