boundarynorm fix (#7553)

* Update utils.py

Proposing update to fix #7014

* Update test_plot.py

added testcase for #7014

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fix as suggested by jklymak

* updated testcase

* enabled type checking of test_discrete_colormap_provided_boundary_norm_matching_cmap_levels()

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* updated whats-new.rst

* enabled type checking of test_discrete_colormap_provided_boundary_norm_matching_cmap_levels(), again

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* Update doc/whats-new.rst

Co-authored-by: Deepak Cherian <[email protected]>

* moved whatsnew for pull 7553 to 2023.04 section

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Deepak Cherian <[email protected]>

Author: 3 people

doc/whats-new.rst

@@ -39,6 +39,8 @@ Bug fixes
   By `Shreyal Gupta <https://github.com/Ravenin7>`_ and `Michael Niklas <https://github.com/headtr1ck>`_.
 - Improve error message when trying to open a file which you do not have permission to read (:issue:`6523`, :pull:`7629`).
   By `Thomas Coleman <https://github.com/ColemanTom>`_.
+- Proper plotting when passing :py:class:`~matplotlib.colors.BoundaryNorm` type argument in :py:meth:`DataArray.plot`. (:issue:`4061`, :issue:`7014`,:pull:`7553`)
+  By `Jelmer Veenstra <https://github.com/veenstrajelmer>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/plot/utils.py

@@ -304,7 +304,7 @@ def _determine_cmap_params(
     if extend is None:
         extend = _determine_extend(calc_data, vmin, vmax)
 
-    if levels is not None or isinstance(norm, mpl.colors.BoundaryNorm):
+    if (levels is not None) and (not isinstance(norm, mpl.colors.BoundaryNorm)):
         cmap, newnorm = _build_discrete_cmap(cmap, levels, extend, filled)
         norm = newnorm if norm is None else norm
 

xarray/tests/test_plot.py

@@ -1204,6 +1204,13 @@ def test_discrete_colormap_provided_boundary_norm(self) -> None:
         primitive = self.darray.plot.contourf(norm=norm)
         np.testing.assert_allclose(primitive.levels, norm.boundaries)
 
+    def test_discrete_colormap_provided_boundary_norm_matching_cmap_levels(
+        self,
+    ) -> None:
+        norm = mpl.colors.BoundaryNorm([0, 5, 10, 15], 4)
+        primitive = self.darray.plot.contourf(norm=norm)
+        assert primitive.colorbar.norm.Ncmap == primitive.colorbar.norm.N
+
 
 class Common2dMixin:
     """