Only allow one of x or y to be specified.

Author: dcherian

xarray/plot/plot.py

@@ -177,11 +177,8 @@ def line(darray, *args, **kwargs):
     hue : string, optional
         Coordinate for which you want multiple lines plotted
         (2D DataArrays only).
-    x : string, optional
-        1D and 2D DataArrays: Coordinate for x axis.
-    y : string, optional
-        1D DataArray: Can be coordinate name or DataArray.name
-        2D DataArray: Coordinate for y axis.
+    x, y : string, optional
+        Coordinates for x, y axis. Only one of these may be specified.
     add_legend : boolean, optional
         Add legend with y axis coordinates (2D inputs only).
     *args, **kwargs : optional
@@ -207,29 +204,27 @@ def line(darray, *args, **kwargs):
 
     ax = get_axis(figsize, size, aspect, ax)
 
-    if ndims == 1:
-        dim, = darray.dims  # get the only dimension name
+    error_msg = 'must be either None or %r' % (' or '.join(darray.dims))
 
-        error_msg = 'must be either None or %r' % dim
-        if darray.name:
-            error_msg += ' or %r.' % darray.name
+    if x not in [None, *darray.dims]:
+        raise ValueError('x ' + error_msg)
 
-        if x not in [None, dim, darray.name]:
-            raise ValueError('x ' + error_msg)
+    if y not in [None, *darray.dims]:
+        raise ValueError('y ' + error_msg)
 
-        if y not in [None, dim, darray.name]:
-            raise ValueError('y ' + error_msg)
+    if x is not None and y is not None:
+        raise ValueError('You cannot specify both x and y kwargs.')
 
-        if x is not None and y is not None and x == y:
-            raise ValueError('Cannot make a plot with x=%r and y=%r' % (x, y))
+    if ndims == 1:
+        dim, = darray.dims  # get the only dimension name
 
-        if (x is None and y is None) or x == dim or y == darray.name:
+        if (x is None and y is None) or x == dim:
             xplt = darray.coords[dim]
             yplt = darray
             xlabel = dim
             ylabel = darray.name
 
-        elif y == dim or x == darray.name:
+        else:
             yplt = darray.coords[dim]
             xplt = darray
             xlabel = darray.name
@@ -246,10 +241,6 @@ def line(darray, *args, **kwargs):
             yplt = darray.transpose(xlabel, huelabel)
 
         else:
-            if x is not None and x is not darray.name:
-                raise ValueError('Cannot make a plot with x=%r and y=%r '
-                                 % (x, y))
-
             ylabel, huelabel = _infer_xy_labels(darray=darray, x=y, y=hue)
             xlabel = darray.name
             xplt = darray.transpose(ylabel, huelabel)

xarray/tests/test_plot.py

@@ -100,22 +100,24 @@ def test_1d_x_y_kw(self):
         da = DataArray(np.cos(z), dims=['z'], coords=[z], name='f')
 
         xy = [[None, None],
-              [None, 'f'],
               [None, 'z'],
-              ['f', None],
-              ['z', None],
-              ['z', 'f'],
-              ['f', 'z']]
+              ['z', None]]
 
         f, ax = plt.subplots(2, 4)
 
         for aa, (x, y) in enumerate(xy):
             da.plot(x=x, y=y, ax=ax.flat[aa])
             ax.flat[aa].set_title('x=' + str(x) + ' | '+'y='+str(y))
 
-        with raises_regex(ValueError, 'Cannot'):
+        with raises_regex(ValueError, 'cannot'):
             da.plot(x='z', y='z')
 
+        with raises_regex(ValueError, 'None'):
+            da.plot(x='f', y='z')
+
+        with raises_regex(ValueError, 'None'):
+            da.plot(x='z', y='f')
+
     def test_2d_line(self):
         with raises_regex(ValueError, 'hue'):
             self.darray[:, :, 0].plot.line()
@@ -126,7 +128,7 @@ def test_2d_line(self):
         self.darray[:, :, 0].plot.line(x='dim_0', hue='dim_1')
         self.darray[:, :, 0].plot.line(y='dim_0', hue='dim_1')
 
-        with raises_regex(ValueError, 'Cannot'):
+        with raises_regex(ValueError, 'cannot'):
             self.darray[:, :, 0].plot.line(x='dim_1', y='dim_0', hue='dim_1')
 
     def test_2d_line_accepts_legend_kw(self):
@@ -323,7 +325,7 @@ def test_ylabel_is_data_name(self):
 
     def test_xlabel_is_data_name(self):
         self.darray.name = 'temperature'
-        self.darray.plot(x=self.darray.name)
+        self.darray.plot(y='period')
         self.assertEqual(self.darray.name, plt.gca().get_xlabel())
 
     def test_format_string(self):