facetgrid: Ensure that colormap params are only determined once.

doc/whats-new.rst

@@ -57,9 +57,11 @@ Bug fixes
 - Fix a regression where deleting a coordinate from a copied :py:class:`DataArray`
   can affect the original :py:class:`Dataarray`.  (:issue:`3899`, :pull:`3871`)
   By `Todd Jennings <https://github.com/toddrjen>`_
+- Fix :py:class:`~xarray.plot.FacetGrid` plots with a single contour. (:issue:`3569`, :pull:`3915`).
+  By `Deepak Cherian <https://github.com/dcherian>`_
 - Use divergent colormap if ``levels`` spans 0. (:issue:`3524`)
   By `Deepak Cherian <https://github.com/dcherian>`_
-- Fix ``FacetGrid`` when ``vmin == vmax``. (:issue:`3734`)
+- Fix :py:class:`~xarray.plot.FacetGrid` when ``vmin == vmax``. (:issue:`3734`)
   By `Deepak Cherian <https://github.com/dcherian>`_
 - Fix bug where plotting line plots with 2D coordinates depended on dimension
   order. (:issue:`3933`)

xarray/plot/facetgrid.py

@@ -273,7 +273,9 @@ def map_dataarray(self, func, x, y, **kwargs):
             # None is the sentinel value
             if d is not None:
                 subset = self.data.loc[d]
-                mappable = func(subset, x=x, y=y, ax=ax, **func_kwargs)
+                mappable = func(
+                    subset, x=x, y=y, ax=ax, **func_kwargs, _is_facetgrid=True
+                )
                 self._mappables.append(mappable)
 
         self._finalize_grid(x, y)

xarray/plot/plot.py

@@ -693,7 +693,10 @@ def newplotfunc(
         _ensure_plottable(xplt, yplt, zval)
 
         cmap_params, cbar_kwargs = _process_cmap_cbar_kwargs(
-            plotfunc, zval.data, **locals()
+            plotfunc,
+            zval.data,
+            **locals(),
+            _is_facetgrid=kwargs.pop("_is_facetgrid", False),
         )
 
         if "contour" in plotfunc.__name__:

xarray/plot/utils.py

@@ -153,6 +153,7 @@ def _determine_cmap_params(
     levels=None,
     filled=True,
     norm=None,
+    _is_facetgrid=False,
 ):
     """
     Use some heuristics to set good defaults for colorbar and range.
@@ -735,6 +736,7 @@ def _process_cmap_cbar_kwargs(
     colors=None,
     cbar_kwargs: Union[Iterable[Tuple[str, Any]], Mapping[str, Any]] = None,
     levels=None,
+    _is_facetgrid=False,
     **kwargs,
 ):
     """
@@ -781,6 +783,12 @@ def _process_cmap_cbar_kwargs(
 
     cmap_args = getfullargspec(_determine_cmap_params).args
     cmap_kwargs.update((a, kwargs[a]) for a in cmap_args if a in kwargs)
-    cmap_params = _determine_cmap_params(**cmap_kwargs)
+    if not _is_facetgrid:
+        cmap_params = _determine_cmap_params(**cmap_kwargs)
+    else:
+        cmap_params = {
+            k: cmap_kwargs[k]
+            for k in ["vmin", "vmax", "cmap", "extend", "levels", "norm"]
+        }
 
     return cmap_params, cbar_kwargs

xarray/tests/test_plot.py

@@ -2319,3 +2319,15 @@ def test_plot_transposes_properly(plotfunc):
     # pcolormesh returns 1D array but imshow returns a 2D array so it is necessary
     # to ravel() on the LHS
     assert np.all(hdl.get_array().ravel() == da.to_masked_array().ravel())
+
+
+@requires_matplotlib
+def test_facetgrid_single_contour():
+    # regression test for GH3569
+    x, y = np.meshgrid(np.arange(12), np.arange(12))
+    z = xr.DataArray(np.sqrt(x ** 2 + y ** 2))
+    z2 = xr.DataArray(np.sqrt(x ** 2 + y ** 2) + 1)
+    ds = xr.concat([z, z2], dim="time")
+    ds["time"] = [0, 1]
+
+    ds.plot.contour(col="time", levels=[4], colors=["k"])