Check cmap type before comparing to categories

hvplot/converter.py

@@ -76,6 +76,7 @@
     process_derived_datetime_pandas,
     _convert_col_names_to_str,
     import_datashader,
+    is_mpl_cmap,
 )
 from .utilities import hvplot_extension
 
@@ -1420,7 +1421,7 @@ def _process_style(self, kwds, plot_opts):
             valid_opts = []
 
         cmap_opts = ('cmap', 'colormap', 'color_key')
-        categories = [
+        categorical_cmaps = [
             'accent',
             'category',
             'dark',
@@ -1485,7 +1486,13 @@ def _process_style(self, kwds, plot_opts):
         if 'color' in style_opts:
             color = style_opts['color']
         elif not isinstance(cmap, dict):
-            if cmap and any(c in cmap for c in categories):
+            # Checks if any of the categorical cmaps matches cmap;
+            # uses any() instead of `cmap in categorical_cmaps` to handle reversed colormaps (suffixed with `_r`).
+            # If cmap is LinearSegmentedColormap, get the name attr, else return the str typed cmap.
+            if (isinstance(cmap, str) or is_mpl_cmap(cmap)) and any(
+                categorical_cmap in getattr(cmap, 'name', cmap)
+                for categorical_cmap in categorical_cmaps
+            ):
                 color = process_cmap(cmap or self._default_cmaps['categorical'], categorical=True)
             else:
                 color = cmap

hvplot/tests/testcharts.py

@@ -606,3 +606,14 @@ def test_table_datetime_index_displayed(self):
 
     def test_table_multi_index_displayed(self):
         raise SkipTest('Dask does not support MultiIndex Dataframes.')
+
+
+def test_cmap_LinearSegmentedColormap():
+    # test for https://github.com/holoviz/hvplot/pull/1461
+    xr = pytest.importorskip('xarray')
+    mpl = pytest.importorskip('matplotlib')
+    import hvplot.xarray  # noqa
+
+    data = np.arange(25).reshape(5, 5)
+    xr_da = xr.DataArray(data)
+    xr_da.hvplot.image(cmap=mpl.colormaps['viridis'])

hvplot/util.py

@@ -750,3 +750,12 @@ def relabel_redim(hv_obj, relabel_kwargs, redim_kwargs):
     if redim_kwargs:
         hv_obj = hv_obj.redim(**redim_kwargs)
     return hv_obj
+
+
+def is_mpl_cmap(obj):
+    """Check if the object is a Matplotlib LinearSegmentedColormap."""
+    if 'matplotlib' not in sys.modules:
+        return False
+    from matplotlib.colors import LinearSegmentedColormap
+
+    return isinstance(obj, LinearSegmentedColormap)