Logic inconsistency in DoubleYAxis.good_colour()

[gb119]

Description:

In src/stonerplots/context/double_y.py:157, the good_colour() method has potentially incorrect logic when self.colours is a string:

match self.colours:
    case list() if -len(self.colours) < axis < len(self.colours):
        return self.colours[axis] is not None
    case str() if -len(self.colours) < axis < len(self.colours):
        return True
    case _:
        return False

When self.colours is a string (e.g., "red"), len(self.colours) returns the string length (3 for "red"), not the number of colours (1). The condition -len(self.colours) < axis < len(self.colours) checks -3 < axis < 3, which may not be the intended behavior.

Questions to Clarify:

1. Should a string colour apply to all axes?
2. Should string length matter, or should it be treated as a single colour?
3. Is the string case meant to handle comma-separated colours?

Suggested Fix (if string should apply to all axes):

def good_colour(self, axis: int) -> bool:
    """Return True if we have a colour defined for this axis."""
    axis = int(axis)
    if self.colours is None:
        return False
    match self.colours:
        case str():  # Single colour string applies to all axes
            return True
        case list() if -len(self.colours) < axis < len(self.colours):
            return self.colours[axis] is not None
        case _:
            return False

[gb119]

We need to handle several cases of colours being a str:

1. Te colours str is actually a valid matplotlib named colour (including the additional named colours introduced by the packages), in which ase the colour should be applied to all axes.
2. The colours iis a string which is a sequence of single letter colour names where each letter specifies the colour of the axes.

At the moment, the case structure handles the second case above, but we should also handle the first. As well as this function we need to make sure the code that actually gets the colours from self.colours follows the same logic. (Ideally we could have helper that does the indexing of self.colours or returns None and good_colour() then just becomes a wrapper that returns False if the indexer returned None.