fix plotting with transposed nondim coords.

Fixes pydata#3138

Author: dcherian

doc/whats-new.rst

@@ -44,6 +44,8 @@ Bug fixes
 - Sync with cftime by removing `dayofwk=-1` for cftime>=1.0.4. 
   By `Anderson Banihirwe <https://github.com/andersy005>`_.
 
+- Fix plotting with transposed 2D non-dimensional coordinates. (:issue:`3138`)
+  By `Deepak Cherian <https://github.com/dcherian>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/plot/plot.py

@@ -11,6 +11,7 @@
 import numpy as np
 import pandas as pd
 
+from ..core.alignment import broadcast
 from .facetgrid import _easy_facetgrid
 from .utils import (
     _add_colorbar,
@@ -666,17 +667,6 @@ def newplotfunc(
             darray=darray, x=x, y=y, imshow=imshow_rgb, rgb=rgb
         )
 
-        # better to pass the ndarrays directly to plotting functions
-        xval = darray[xlab].values
-        yval = darray[ylab].values
-
-        # check if we need to broadcast one dimension
-        if xval.ndim < yval.ndim:
-            xval = np.broadcast_to(xval, yval.shape)
-
-        if yval.ndim < xval.ndim:
-            yval = np.broadcast_to(yval, xval.shape)
-
         # May need to transpose for correct x, y labels
         # xlab may be the name of a coord, we have to check for dim names
         if imshow_rgb:
@@ -690,8 +680,17 @@ def newplotfunc(
         elif darray[xlab].dims[-1] == darray.dims[0]:
             darray = darray.transpose(transpose_coords=True)
 
-        # Pass the data as a masked ndarray too
-        zval = darray.to_masked_array(copy=False)
+        # better to pass the ndarrays directly to plotting functions
+        # Pass the data as a masked ndarray
+        if darray[xlab].ndim == 1 and darray[ylab].ndim == 1:
+            xval = darray[xlab].values
+            yval = darray[ylab].values
+            zval = darray.to_masked_array(copy=False)
+        else:
+            xval, yval, zval = map(
+                lambda x: x.values, broadcast(darray[xlab], darray[ylab], darray)
+            )
+            zval = np.ma.masked_array(zval, mask=pd.isnull(zval), copy=False)
 
         # Replace pd.Intervals if contained in xval or yval.
         xplt, xlab_extra = _resolve_intervals_2dplot(xval, plotfunc.__name__)

xarray/tests/test_plot.py

@@ -2145,3 +2145,19 @@ def test_yticks_kwarg(self, da):
         da.plot(yticks=np.arange(5))
         expected = np.arange(5)
         assert np.all(plt.gca().get_yticks() == expected)
+
+
+@requires_matplotlib
+@pytest.mark.parametrize("plotfunc", ["pcolormesh", "contourf", "contour"])
+def test_plot_transposed_nondim_coord(plotfunc):
+    x = np.linspace(0, 10, 101)
+    h = np.linspace(3, 7, 101)
+    s = np.linspace(0, 1, 51)
+    z = s[:, np.newaxis] * h[np.newaxis, :]
+    da = xr.DataArray(
+        np.sin(x) * np.cos(z),
+        dims=["s", "x"],
+        coords={"x": x, "s": s, "z": (("s", "x"), z), "zt": (("x", "s"), z.T)},
+    )
+    getattr(da.plot, plotfunc)(x="x", y="zt")
+    getattr(da.plot, plotfunc)(x="zt", y="x")