Allow RGB[A] dim for imshow to be in any order

Includes new `rgb` keyword to tell imshow about that dimension, and much
error handling in inference.

xarray/plot/facetgrid.py

@@ -239,8 +239,9 @@ def map_dataarray(self, func, x, y, **kwargs):
         func_kwargs.update({'add_colorbar': False, 'add_labels': False})
 
         # Get x, y labels for the first subplot
-        x, y = _infer_xy_labels(darray=self.data.loc[self.name_dicts.flat[0]],
-                                x=x, y=y, imshow=func.__name__ == 'imshow')
+        x, y = _infer_xy_labels(
+            darray=self.data.loc[self.name_dicts.flat[0]], x=x, y=y,
+            imshow=func.__name__ == 'imshow', rgb=kwargs.get('rgb', None))
 
         for d, ax in zip(self.name_dicts.flat, self.axes.flat):
             # None is the sentinel value

xarray/plot/plot.py

@@ -468,13 +468,13 @@ def newplotfunc(darray, x=None, y=None, figsize=None, size=None,
                           "Use colors keyword instead.",
                           DeprecationWarning, stacklevel=3)
 
-        xlab, ylab = _infer_xy_labels(darray=darray, x=x, y=y,
-                                      imshow=plotfunc.__name__ == 'imshow')
+        rgb = kwargs.pop('rgb', None)
+        xlab, ylab = _infer_xy_labels(
+            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
-        zval = darray.to_masked_array(copy=False)
 
         # check if we need to broadcast one dimension
         if xval.ndim < yval.ndim:
@@ -485,8 +485,19 @@ def newplotfunc(darray, x=None, y=None, figsize=None, size=None,
 
         # 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 darray[xlab].dims[-1] == darray.dims[0]:
-            zval = zval.T
+        if imshow_rgb:
+            # For RGB[A] images, matplotlib requires the color dimension
+            # to be last.  In Xarray the order should be unimportant, so
+            # we transpose to (y, x, color) to make this work.
+            yx_dims = (ylab, xlab)
+            dims = yx_dims + tuple(d for d in darray.dims if d not in yx_dims)
+            if dims != darray.dims:
+                darray = darray.transpose(*dims)
+        elif darray[xlab].dims[-1] == darray.dims[0]:
+            darray = darray.transpose()
+
+        # Pass the data as a masked ndarray too
+        zval = darray.to_masked_array(copy=False)
 
         _ensure_plottable(xval, yval)
 
@@ -591,8 +602,9 @@ def imshow(x, y, z, ax, **kwargs):
     Wraps :func:`matplotlib:matplotlib.pyplot.imshow`
 
     While other plot methods require the DataArray to be strictly
-    two-dimensional, ``imshow`` also accepts a 3D array where the third
-    dimension can be interpreted as RGB or RGBA color channels.
+    two-dimensional, ``imshow`` also accepts a 3D array where some
+    dimension can be interpreted as RGB or RGBA color channels and
+    allows this dimension to be specified via the kwarg ``rgb=``.
     In this case, ``robust=True`` will saturate the image in the
     usual way, consistenly between all bands and facets.
 

xarray/plot/utils.py

@@ -254,21 +254,72 @@ def _determine_cmap_params(plot_data, vmin=None, vmax=None, cmap=None,
                 levels=levels, norm=norm)
 
 
-def _infer_xy_labels(darray, x, y, imshow=False):
+def _infer_xy_labels_3d(darray, x, y, rgb):
+    """
+    Determine x and y labels for showing RGB images.
+
+    Attempts to infer which dimension is RGB/RGBA by size and order of dims.
+
+    """
+    # Start by detecting and reporting invalid combinations of arguments
+    assert darray.ndim == 3
+    not_none = [a for a in (x, y, rgb) if a is not None]
+    if len(set(not_none)) < len(not_none):
+        raise ValueError('Dimensions passed as x, y, and rgb must be unique.')
+    for label in not_none:
+        if label not in darray.dims:
+            raise ValueError('%r is not a dimension' % (label,))
+
+    # Then calculate rgb dimension if certain and check validity
+    could_be_color = [label for label in darray.dims
+                      if darray[label].size in (3, 4) and label not in (x, y)]
+    if rgb is None and not could_be_color:
+        raise ValueError(
+            'A 3-dimensional array was passed to imshow(), but there is no '
+            'dimension that could be color.  At least one dimension must be '
+            'of size 3 (RGB) or 4 (RGBA), and not given as x or y.')
+    if rgb is None and len(could_be_color) == 1:
+        rgb = could_be_color[0]
+    if rgb is not None and darray[rgb].size not in (3, 4):
+        raise ValueError('Cannot interpret dim %r of size %s as RGB or RGBA.'
+                         % (rgb, darray[rgb].size))
+
+    # If rgb dimension is still unknown, there must be two or three dimensions
+    # in could_be_color.  We therefore warn, and use a heuristic to break ties.
+    if rgb is None:
+        assert len(could_be_color) in (2, 3)
+        if darray.dims[-1] in could_be_color:
+            rgb = darray.dims[-1]
+            warnings.warn(
+                'Several dimensions of this array could be colors.  Xarray '
+                'will use the last dimension (%r) to match '
+                'matplotlib.pyplot.imshow.  You can pass names of x, y, '
+                'and/or rgb dimensions to override this guess.' % rgb)
+        else:
+            rgb = darray.dims[0]
+            warnings.warn(
+                '%r has been selected as the color dimension, but %r would '
+                'also be valid.  Pass names of x, y, and/or rgb dimensions to '
+                'override this guess.' % darray.dims[:2])
+    assert rgb is not None
+
+    # Finally, we pick out the red slice and delegate to the 2D version:
+    return _infer_xy_labels(darray.isel(**{rgb: 0}).squeeze(), x, y)
+
+
+def _infer_xy_labels(darray, x, y, imshow=False, rgb=None):
     """
     Determine x and y labels. For use in _plot2d
 
     darray must be a 2 dimensional data array, or 3d for imshow only.
     """
+    if imshow and darray.ndim == 3:
+        return _infer_xy_labels_3d(darray, x, y, rgb)
 
     if x is None and y is None:
         if darray.ndim != 2:
-            if not imshow:
-                raise ValueError('DataArray must be 2d')
-            elif darray.ndim != 3 or darray.shape[2] not in (3, 4):
-                raise ValueError('DataArray for imshow must be 2d, MxNx3 for '
-                                 'RGB image, or MxNx4 for RGBA image.')
-        y, x, *_ = darray.dims
+            raise ValueError('DataArray must be 2d')
+        y, x = darray.dims
     elif x is None:
         if y not in darray.dims:
             raise ValueError('y must be a dimension name if x is not supplied')

xarray/tests/test_plot.py

@@ -1040,6 +1040,21 @@ def test_plot_rgb_faceted(self):
         ).plot.imshow(row='a', col='b')
         self.assertEqual(0, len(find_possible_colorbars()))
 
+    def test_plot_rgba_image_transposed(self):
+        # We can handle the color axis being in any position
+        DataArray(
+            easy_array((4, 10, 15), start=0),
+            dims=['band', 'y', 'x'],
+        ).plot.imshow()
+
+    def test_warns_ambigious_dim(self):
+        arr = DataArray(easy_array((3, 3, 3)), dims=['y', 'x', 'band'])
+        with pytest.warns(UserWarning):
+            arr.plot.imshow()
+        # but doesn't warn if dimensions specified
+        arr.plot.imshow(rgb='band')
+        arr.plot.imshow(x='x', y='y')
+
 
 class TestFacetGrid(PlotTestCase):