include max value in pcolormesh output when levels are requested

[JJFlorian]

• Closes Maximum value not always included in pcolormesh output when levels are requested #10911

Not the most elegant solution, but it works and I couldn't figure out another way of solving this for now. The test_discrete_colormap_list_of_levels test fails now, but I would say that it is safe to use numpy's assert_array_almost_equal.

If its not, I'll try to find another solution.

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[kkollsga]

Nice find — the nextafter nudge is the right idea. I think the fix can be made a bit more targeted though, which would avoid weakening the existing test.

The root cause is in _build_discrete_cmap, not _determine_cmap_params — it's specifically BoundaryNorm's half-open intervals [a, b) that exclude vmax. If we nudge the norm's boundaries there instead of the levels themselves, cmap_params["levels"] stays clean and the existing assert_array_equal test still passes:

def _build_discrete_cmap(cmap, levels, extend, filled):
    ...
    new_cmap, cnorm = mpl.colors.from_levels_and_colors(levels, pal, extend=extend)

    # BoundaryNorm uses half-open intervals [a, b), so values exactly at the
    # last boundary are excluded. When there's no "over" extension, nudge the
    # upper boundary so vmax is included in the last bin. (GH10911)
    if extend in ("neither", "min"):
        boundaries = cnorm.boundaries.copy()
        boundaries[-1] = np.nextafter(boundaries[-1], np.inf)
        cnorm = mpl.colors.BoundaryNorm(boundaries, cnorm.Ncmap)

    ...

The condition checks extend rather than comparing against calc_data — when extend has no "over" handling, there's no color assigned for values beyond the last boundary, so we need the last bin to be inclusive. When extend is "max" or "both", values above the last boundary already get the over-color, so the nudge isn't needed.