Fix contour when levels is scalar and norm is provided. (#3914)

Fixes #3735

Author: dcherian

doc/whats-new.rst

@@ -91,6 +91,8 @@ Bug fixes
   By `Deepak Cherian <https://github.com/dcherian>`_
 - Fix :py:class:`~xarray.plot.FacetGrid` when ``vmin == vmax``. (:issue:`3734`)
   By `Deepak Cherian <https://github.com/dcherian>`_
+- Fix plotting when ``levels`` is a scalar and ``norm`` is provided. (:issue:`3735`)
+  By `Deepak Cherian <https://github.com/dcherian>`_
 - Fix bug where plotting line plots with 2D coordinates depended on dimension
   order. (:issue:`3933`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.

xarray/plot/utils.py

@@ -268,7 +268,7 @@ def _determine_cmap_params(
             cmap = OPTIONS["cmap_sequential"]
 
     # Handle discrete levels
-    if levels is not None and norm is None:
+    if levels is not None:
         if is_scalar(levels):
             if user_minmax:
                 levels = np.linspace(vmin, vmax, levels)

xarray/tests/test_plot.py

@@ -854,21 +854,22 @@ def test_norm_sets_vmin_vmax(self):
         vmin = self.data.min()
         vmax = self.data.max()
 
-        for norm, extend in zip(
+        for norm, extend, levels in zip(
             [
+                mpl.colors.Normalize(),
                 mpl.colors.Normalize(),
                 mpl.colors.Normalize(vmin + 0.1, vmax - 0.1),
                 mpl.colors.Normalize(None, vmax - 0.1),
                 mpl.colors.Normalize(vmin + 0.1, None),
             ],
-            ["neither", "both", "max", "min"],
+            ["neither", "neither", "both", "max", "min"],
+            [7, None, None, None, None],
         ):
 
             test_min = vmin if norm.vmin is None else norm.vmin
             test_max = vmax if norm.vmax is None else norm.vmax
 
-            cmap_params = _determine_cmap_params(self.data, norm=norm)
-
+            cmap_params = _determine_cmap_params(self.data, norm=norm, levels=levels)
             assert cmap_params["vmin"] == test_min
             assert cmap_params["vmax"] == test_max
             assert cmap_params["extend"] == extend