Improved handling for datashading empty objects

holoviews/core/util.py

@@ -1569,14 +1569,17 @@ def dt_to_int(value, time_unit='us'):
     """
     Converts a datetime type to an integer with the supplied time unit.
     """
-    if time_unit == 'ns':
-        tscale = 1
+    if isinstance(value, np.datetime64):
+        value = np.datetime64(value, 'ns')
+        tscale = np.timedelta64(1, time_unit)/np.timedelta64(1, 'ns') * 1000
+    elif time_unit == 'ns':
+        tscale = 1000.
     else:
         tscale = 1./np.timedelta64(1, time_unit).tolist().total_seconds()
     if pd and isinstance(value, pd.Timestamp):
         value = value.to_pydatetime()
     elif isinstance(value, np.datetime64):
-        value = np.datetime64(value, 'ns').tolist()
+        value = value.tolist()
     if isinstance(value, (int, long)):
         # Handle special case of nanosecond precision which cannot be
         # represented by python datetime

holoviews/operation/datashader.py

@@ -255,7 +255,7 @@ def get_agg_data(cls, obj, category=None):
                 paths = [p.compute() if isinstance(p, dd.DataFrame) else p for p in paths]
                 df = pd.concat(paths)
         else:
-            df = paths[0]
+            df = paths[0] if paths else pd.DataFrame([], columns=[x.name, y.name])
         if category and df[category].dtype.name != 'category':
             df[category] = df[category].astype('category')
 
@@ -360,6 +360,10 @@ def _process(self, element, key=None):
             xarray = xr.DataArray(np.full((height, width), np.NaN, dtype=np.float32),
                                   dims=['y', 'x'], coords={'x': xs, 'y': ys})
             return self.p.element_type(xarray)
+        elif not len(data):
+            xarray = xr.DataArray(np.full((height, width), np.NaN, dtype=np.float32),
+                                  dims=[y.name, x.name], coords={x.name: xs, y.name: ys})
+            return self.p.element_type(xarray)
 
         cvs = ds.Canvas(plot_width=width, plot_height=height,
                         x_range=x_range, y_range=y_range)

tests/testcoreutils.py

@@ -562,10 +562,18 @@ def test_datetime64_to_us_int(self):
         dt = np.datetime64(datetime.datetime(2017, 1, 1))
         self.assertEqual(dt_to_int(dt), 1483228800000000.0)
 
+    def test_datetime64_us_to_us_int(self):
+        dt = np.datetime64(datetime.datetime(2017, 1, 1), 'us')
+        self.assertEqual(dt_to_int(dt), 1483228800000000.0)
+
+    def test_datetime64_s_to_us_int(self):
+        dt = np.datetime64(datetime.datetime(2017, 1, 1), 's')
+        self.assertEqual(dt_to_int(dt), 1483228800000000.0)
+
     def test_timestamp_to_us_int(self):
         dt = pd.Timestamp(datetime.datetime(2017, 1, 1))
         self.assertEqual(dt_to_int(dt), 1483228800000000.0)
-    
+
     def test_datetime_to_s_int(self):
         dt = datetime.datetime(2017, 1, 1)
         self.assertEqual(dt_to_int(dt, 's'), 1483228800.0)
@@ -574,6 +582,14 @@ def test_datetime64_to_s_int(self):
         dt = np.datetime64(datetime.datetime(2017, 1, 1))
         self.assertEqual(dt_to_int(dt, 's'), 1483228800.0)
 
+    def test_datetime64_us_to_s_int(self):
+        dt = np.datetime64(datetime.datetime(2017, 1, 1), 'us')
+        self.assertEqual(dt_to_int(dt, 's'), 1483228800.0)
+
+    def test_datetime64_s_to_s_int(self):
+        dt = np.datetime64(datetime.datetime(2017, 1, 1), 's')
+        self.assertEqual(dt_to_int(dt, 's'), 1483228800.0)
+
     def test_timestamp_to_s_int(self):
         dt = pd.Timestamp(datetime.datetime(2017, 1, 1))
         self.assertEqual(dt_to_int(dt, 's'), 1483228800.0)