plot: If provided with colormap do not modify it.

doc/whats-new.rst

@@ -37,6 +37,8 @@ Bug fixes
   By `Mayeul d'Avezac <https://github.com/mdavezac>`_.
 - Return correct count for scalar datetime64 arrays (:issue:`2770`)
   By `Dan Nowacki <https://github.com/dnowacki-usgs>`_.
+- Fix facetgrid colormap bug when ``extend=True``. (:issue:`2932`)
+  By `Deepak Cherian <https://github.com/dcherian`_.
 - A deep copy deep-copies the coords (:issue:`1463`)
   By `Martin Pletcher <https://github.com/pletchm>`_.
 

xarray/plot/utils.py

@@ -265,7 +265,8 @@ def _determine_cmap_params(plot_data, vmin=None, vmax=None, cmap=None,
     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 or isinstance(norm, mpl.colors.BoundaryNorm))
+            and (not isinstance(cmap, mpl.colors.Colormap))):
         cmap, newnorm = _build_discrete_cmap(cmap, levels, extend, filled)
         norm = newnorm if norm is None else norm
 

xarray/tests/test_plot.py

@@ -4,12 +4,10 @@
 import numpy as np
 import pandas as pd
 import pytest
-from numpy.testing import assert_array_equal
 
 import xarray as xr
 import xarray.plot as xplt
 from xarray import DataArray
-from xarray.coding.times import _import_cftime
 from xarray.plot.plot import _infer_interval_breaks
 from xarray.plot.utils import (
     _build_discrete_cmap, _color_palette, _determine_cmap_params,
@@ -539,6 +537,25 @@ def test_cmap_sequential_option(self):
             cmap_params = _determine_cmap_params(self.data)
             assert cmap_params['cmap'] == 'magma'
 
+    def test_do_nothing_if_provided_cmap(self):
+        cmap_list = [
+            mpl.colors.LinearSegmentedColormap.from_list('name', ['r', 'g']),
+            mpl.colors.ListedColormap(['r', 'g', 'b'])
+        ]
+
+        # can't parametrize with mpl objects when mpl is absent
+        for cmap in cmap_list:
+            cmap_params = _determine_cmap_params(self.data,
+                                                 cmap=cmap,
+                                                 levels=7)
+            assert cmap_params['cmap'] is cmap
+
+    def test_do_something_if_provided_str_cmap(self):
+        cmap = 'RdBu_r'
+        cmap_params = _determine_cmap_params(self.data, cmap=cmap, levels=7)
+        assert cmap_params['cmap'] is not cmap
+        assert isinstance(cmap_params['cmap'], mpl.colors.ListedColormap)
+
     def test_cmap_sequential_explicit_option(self):
         with xr.set_options(cmap_sequential=mpl.cm.magma):
             cmap_params = _determine_cmap_params(self.data)