Guess the complementary dimension when only one is passed to pcolormesh

doc/whats-new.rst

@@ -22,6 +22,10 @@ v0.9.2 (unreleased)
 Enhancements
 ~~~~~~~~~~~~
 
+- When `plot()` is called on a 2D DataArray and only one dimension is
+  specified with `x=` or `y=`, the other dimension is now guessed. By
+  `Vincent Noel <https://github.com/vnoel>`_.
+
 Bug fixes
 ~~~~~~~~~
 - ``rolling`` now keeps its original dimension order (:issue:`1125`).

xarray/plot/utils.py

@@ -221,8 +221,14 @@ def _infer_xy_labels(darray, x, y):
         if darray.ndim != 2:
             raise ValueError('DataArray must be 2d')
         y, x = darray.dims
-    elif x is None or y is None:
-        raise ValueError('cannot supply only one of x and y')
+    elif x is None:
+        if y not in darray.dims:
+            raise ValueError('y must be a dimension name if x is not supplied')
+        x = darray.dims[0] if y == darray.dims[1] else darray.dims[1]
+    elif y is None:
+        if x not in darray.dims:
+            raise ValueError('x must be a dimension name if y is not supplied')
+        y = darray.dims[0] if x == darray.dims[1] else darray.dims[1]
     elif any(k not in darray.coords and k not in darray.dims for k in (x, y)):
         raise ValueError('x and y must be coordinate variables')
     return x, y

xarray/tests/test_plot.py

@@ -636,17 +636,27 @@ def test_xy_strings(self):
         self.assertEqual('x', ax.get_ylabel())
 
     def test_positional_coord_string(self):
-        with self.assertRaisesRegexp(ValueError, 'cannot supply only one'):
-            self.plotmethod('y')
-        with self.assertRaisesRegexp(ValueError, 'cannot supply only one'):
-            self.plotmethod(y='x')
+        self.plotmethod(y='x')
+        ax = plt.gca()
+        self.assertEqual('x', ax.get_ylabel())
+        self.assertEqual('y', ax.get_xlabel())
+
+        self.plotmethod(x='x')
+        ax = plt.gca()
+        self.assertEqual('x', ax.get_xlabel())
+        self.assertEqual('y', ax.get_ylabel())
 
     def test_bad_x_string_exception(self):
-        with self.assertRaisesRegexp(ValueError, 'x and y must be coordinate'):
+        with self.assertRaisesRegexp(
+                ValueError, 'x and y must be coordinate variables'):
             self.plotmethod('not_a_real_dim', 'y')
+        with self.assertRaisesRegexp(
+                ValueError, 'x must be a dimension name if y is not supplied'):
+            self.plotmethod(x='not_a_real_dim')
+        with self.assertRaisesRegexp(
+                ValueError, 'y must be a dimension name if x is not supplied'):
+            self.plotmethod(y='not_a_real_dim')
         self.darray.coords['z'] = 100
-        with self.assertRaisesRegexp(ValueError, 'cannot supply only one'):
-            self.plotmethod('z')
 
     def test_coord_strings(self):
         # 1d coords (same as dims)