make plotting work with transposed nondim coords.

doc/whats-new.rst

@@ -76,6 +76,9 @@ Bug fixes
   :py:meth:`xarray.core.groupby.DatasetGroupBy.reduce` when reducing over multiple dimensions.
   (:issue:`3402`). By `Deepak Cherian <https://github.com/dcherian/>`_
 
+- Fix plotting with transposed 2D non-dimensional coordinates. (:issue:`3138`)
+  By `Deepak Cherian <https://github.com/dcherian>`_.
+
 Documentation
 ~~~~~~~~~~~~~
 - Fix leap year condition in example (http://xarray.pydata.org/en/stable/examples/monthly-means.html) by `Mickaël Lalande <https://github.com/mickaellalande>`_.

xarray/plot/plot.py

@@ -672,10 +672,16 @@ def newplotfunc(
 
         # check if we need to broadcast one dimension
         if xval.ndim < yval.ndim:
-            xval = np.broadcast_to(xval, yval.shape)
+            if xval.shape[0] == yval.shape[0]:
+                xval = np.broadcast_to(xval[:, np.newaxis], yval.shape)
+            else:
+                xval = np.broadcast_to(xval[np.newaxis, :], yval.shape)
 
-        if yval.ndim < xval.ndim:
-            yval = np.broadcast_to(yval, xval.shape)
+        elif yval.ndim < xval.ndim:
+            if yval.shape[0] == xval.shape[0]:
+                yval = np.broadcast_to(yval[:, np.newaxis], xval.shape)
+            else:
+                yval = np.broadcast_to(yval[np.newaxis, :], 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
@@ -687,7 +693,7 @@ def newplotfunc(
             dims = yx_dims + tuple(d for d in darray.dims if d not in yx_dims)
             if dims != darray.dims:
                 darray = darray.transpose(*dims, transpose_coords=True)
-        elif darray[xlab].dims[-1] == darray.dims[0]:
+        elif xval.shape[-1] == darray.shape[0]:
             darray = darray.transpose(transpose_coords=True)
 
         # Pass the data as a masked ndarray too

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")