stairs with dashed linestyle and fill=True raises ValueError #31302

If you change test_stairs_options to mpl20 style, then it crashes. What we have worked out is that it is due to artist "G", which sets ls='--', fill=True, but does not set a linewidth.

The setup for stairs sets a default linewidth=0 if fill=True. The difference between classic and mpl20 styles is that the latter performs linewidth-scaling on the linestyle. So in mpl20 style, the dashes are scaled to 0, which raises in the backend.

__________________ test_stairs_options[png] ___________________

args = (), kwds = {'extension': 'png', 'request': <FixtureRequest for <Function test_stairs_options[png]>>}

    @wraps(func)
    def inner(*args, **kwds):
        with self._recreate_cm():
          return func(*args, **kwds)

/usr/lib64/python3.13/contextlib.py:85: 
 _ _ _ _ _ _ _ _ _ _ _ _
lib/matplotlib/figure.py:3508: in savefig
    self.canvas.print_figure(fname, **kwargs)
lib/matplotlib/backend_bases.py:2275: in print_figure
    result = print_method(
lib/matplotlib/backend_bases.py:2132: in <lambda>
    print_method = functools.wraps(meth)(lambda *args, **kwargs: meth(
lib/matplotlib/backends/backend_agg.py:539: in print_png
    self._print_pil(filename_or_obj, "png", pil_kwargs, metadata)
lib/matplotlib/backends/backend_agg.py:487: in _print_pil
    FigureCanvasAgg.draw(self)
lib/matplotlib/backends/backend_agg.py:440: in draw
    self.figure.draw(self.renderer)
lib/matplotlib/artist.py:94: in draw_wrapper
    result = draw(artist, renderer, *args, **kwargs)
lib/matplotlib/artist.py:71: in draw_wrapper
    return draw(artist, renderer)
lib/matplotlib/figure.py:3275: in draw
    mimage._draw_list_compositing_images(
lib/matplotlib/image.py:133: in _draw_list_compositing_images
    a.draw(renderer)
lib/matplotlib/artist.py:71: in draw_wrapper
    return draw(artist, renderer)
lib/matplotlib/axes/_base.py:3282: in draw
    mimage._draw_list_compositing_images(
lib/matplotlib/image.py:133: in _draw_list_compositing_images
    a.draw(renderer)
lib/matplotlib/artist.py:71: in draw_wrapper
    return draw(artist, renderer)
lib/matplotlib/patches.py:740: in draw
    self._draw_paths_with_artist_properties(
lib/matplotlib/patches.py:723: in _draw_paths_with_artist_properties
    gc.set_dashes(*self._dash_pattern)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <matplotlib.backend_bases.GraphicsContextBase object at 0x7f3016ba3750>, dash_offset = 0.0, dash_list = [0.0, 0.0]

    def set_dashes(self, dash_offset, dash_list):
        """
        Set the dash style for the gc.
    
        Parameters
        ----------
        dash_offset : float
            Distance, in points, into the dash pattern at which to
            start the pattern. It is usually set to 0.
        dash_list : array-like or None
            The on-off sequence as points.  None specifies a solid line. All
            values must otherwise be non-negative (:math:`\\ge 0`).
    
        Notes
        -----
        See p. 666 of the PostScript
        `Language Reference
        <https://www.adobe.com/jp/print/postscript/pdfs/PLRM.pdf>`_
        for more info.
        """
        if dash_list is not None:
            dl = np.asarray(dash_list)
            if np.any(dl < 0.0):
                raise ValueError(
                    "All values in the dash list must be non-negative")
            if dl.size and not np.any(dl > 0.0):
              raise ValueError(
                    'At least one value in the dash list must be positive')
E               ValueError: At least one value in the dash list must be positive

lib/matplotlib/backend_bases.py:897: ValueError

Originally posted by @QuLogic in #31300