Support Cn, n>9 in plot() shorthand format.

[anntzer]

PR summary

Closes #27828.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[ksunden]

It is a corner case that I'm not too sure we are worried about, but does induce a change in behavior:

plot([2, 5, 1], "-C11") is currently interpreted as ls=-, c=C1, ms=1 ("tri-down" marker... which is unfortunately represented by a digit) 1,2,3,4, (tri in each direction) and 8 (octagon) each have markers associated with their string representations, so it is not only tri-down (1).

With this PR it will be interpreted as a solid line of color C11, with no marker.

A workaround would be to specify the marker anywhere other than immediately after the color specification (1-C1, -1C1, C1-1 should all work).

I will also note that C11 goes through the fast path and does not get a marker, so there is already some inconsistency if you are using the tri-down marker which induces order dependence. (And so this only matters in the case where you also want a line and not just markers)

How concerned are we with this case?

Personally I'm of the opinion that color cycles are allowed to be longer than 10 colors, and we should support specifying them here, and thus I think I'm okay with it, but do want it acknowledged. (I'm also wondering if that change of behavior, however slight, requires a release note)

Other than the behavior change, the implementation looks good.

[timhoffm]

I’m ok with the change. The old interpretation of “C11” is not intuitive. Ideally, people will already have used a different order, at least I would have done that. But anyway it’s quite a niche case and not worth blocking the CN extension.

[tacaswell]

It needs a clear API change not, but I'm also onboard with this change.

[anntzer]

Added API change note.