Ensure empty aggregate does not drop datetimes in shade (#4038)

holoviews/operation/datashader.py

@@ -11,6 +11,7 @@
 import datashader.reductions as rd
 import datashader.transfer_functions as tf
 import dask.dataframe as dd
+
 from param.parameterized import bothmethod
 
 try:
@@ -20,7 +21,7 @@
     hammer_bundle, connect_edges = object, object
 
 from ..core import (Operation, Element, Dimension, NdOverlay,
-                    CompositeOverlay, Dataset, Overlay)
+                    CompositeOverlay, Dataset, Overlay, OrderedDict)
 from ..core.data import PandasInterface, XArrayInterface, DaskInterface
 from ..core.util import (
     LooseVersion, basestring, cftime_types, cftime_to_timestamp,
@@ -1204,6 +1205,16 @@ def uint32_to_uint8(cls, img):
         return np.flipud(img.view(dtype=np.uint8).reshape(img.shape + (4,)))
 
 
+    @classmethod
+    def uint32_to_uint8_xr(cls, img):
+        """
+        Cast uint32 xarray DataArray to 4 uint8 channels.
+        """
+        new_array = img.values.view(dtype=np.uint8).reshape(img.shape + (4,))
+        coords = OrderedDict(list(img.coords.items())+[('band', [0, 1, 2, 3])])
+        return xr.DataArray(new_array, coords=coords, dims=img.dims+('band',))
+
+
     @classmethod
     def rgb2hex(cls, rgb):
         """
@@ -1278,10 +1289,6 @@ def _process(self, element, key=None):
         elif ds_version > '0.5.0' and self.p.normalization != 'eq_hist':
             shade_opts['span'] = element.range(vdim)
 
-        for d in kdims:
-            if array[d.name].dtype.kind == 'M':
-                array[d.name] = array[d.name].astype('datetime64[us]').astype('int64')
-
         with warnings.catch_warnings():
             warnings.filterwarnings('ignore', r'invalid value encountered in true_divide')
             if np.isnan(array.data).all():
@@ -1293,7 +1300,7 @@ def _process(self, element, key=None):
         params = dict(get_param_values(element), kdims=kdims,
                       bounds=bounds, vdims=RGB.vdims[:],
                       xdensity=xdensity, ydensity=ydensity)
-        return RGB(self.uint32_to_uint8(img.data), **params)
+        return RGB(self.uint32_to_uint8_xr(img), **params)
 
 
 

holoviews/tests/operation/testdatashader.py

@@ -442,6 +442,18 @@ def test_shade_categorical_images_grid(self):
                        vdims=RGB.vdims+[Dimension('A', range=(0, 1))])
         self.assertEqual(shaded, expected)
 
+    def test_shade_dt_xaxis_constant_yaxis(self):
+        df = pd.DataFrame({'y': np.ones(100)}, index=pd.date_range('1980-01-01', periods=100, freq='1T'))
+        rgb = shade(rasterize(Curve(df), dynamic=False))
+        xs = np.array(['1980-01-01T00:16:30.000000', '1980-01-01T00:49:30.000000',
+                       '1980-01-01T01:22:30.000000'], dtype='datetime64[us]')
+        ys = np.array([])
+        bounds = (np.datetime64('1980-01-01T00:00:00.000000'), 1.0,
+                  np.datetime64('1980-01-01T01:39:00.000000'), 1.0)
+        expected = RGB((xs, ys, np.empty((0, 3, 4))), ['index', 'y'],
+                       xdensity=1, ydensity=1, bounds=bounds)
+        self.assertEqual(rgb, expected)
+
 
 
 class DatashaderRegridTests(ComparisonTestCase):