Description
Problem
The current set of hatching options within matplotlib is somewhat limited, being only the following, with the ability to make more dense ones.
In geoscience (and possibly other fields, but that is my background) there are a number of other hatching patterns that have long historical use and are still common, cf the USGS templates, available as a pdf here. Some of these are complex enough to be difficult to generate programmatically (eg 706: contorted schist), while others are much simpler (eg 328-K, which is a grid of offset +).
Is there a means to safely define hatches such the latter? A workaround, adding custom hatches to _hatch_types can be done but raises a deprecation warning (MatplotlibDeprecationWarning: hatch must consist of a string of "*+-./OX\ox|" or None, but found the following invalid values "p". Passing invalid values is deprecated since 3.4 and will become an error two minor releases later.) since PR #17926, and is therefore not a stable long-term solution.
Proposed Solution
It would be good to have a means to define custom hatches in a safe way. In many cases I think it likely to be package maintainers adding the functionality (as striplog does) rather than users.
A less comprehensive solution that would still cover many common use-cases would be to allow for repetitive hatching of a single (user-chosen) symbol. (That is, being able to place a regular grid of + or L in the way that the current hatches API handles cases like . and o.)