Heat map: Cannot cluster a single instance

Orange/widgets/visualize/owheatmap.py

@@ -318,11 +318,11 @@ def __new__(cls, title, indices, sortindices, cluster=None,
                                cluster, cluster_ordered)
 
     @property
-    def is_empty(self):
+    def can_cluster(self):
         if isinstance(self.indices, slice):
-            return (self.indices.stop - self.indices.start) == 0
+            return (self.indices.stop - self.indices.start) > 1
         else:
-            return len(self.indices) == 0
+            return len(self.indices) > 1
 
     @property
     def cluster_ord(self):
@@ -794,7 +794,7 @@ def cluster_rows(self, data, parts):
             else:
                 cluster_ord = None
 
-            if not row.is_empty:
+            if row.can_cluster:
                 need_dist = cluster is None or cluster_ord is None
                 if need_dist:
                     subset = data[row.indices]

Orange/widgets/visualize/tests/test_owheatmap.py

@@ -5,7 +5,7 @@
 import numpy as np
 from sklearn.exceptions import ConvergenceWarning
 
-from Orange.data import Table, Domain, ContinuousVariable
+from Orange.data import Table, Domain, ContinuousVariable, DiscreteVariable
 from Orange.preprocess import Continuize
 from Orange.widgets.visualize.owheatmap import OWHeatMap
 from Orange.widgets.tests.base import WidgetTest, WidgetOutputsTestMixin, datasets
@@ -168,3 +168,10 @@ def test_use_enough_colors(self):
             colors[r] = c.red(), c.green(), c.blue()
         unique_colors = len(np.unique(colors, axis=0))
         self.assertLessEqual(len(data)*self.widget.threshold_low, unique_colors)
+
+    def test_cls_with_single_instance(self):
+        table = Table(Domain([ContinuousVariable("c1")],
+                             [DiscreteVariable("c2", values=["a", "b"])]),
+                      np.array([[1], [2], [3]]), np.array([[0], [0], [1]]))
+        self.send_signal(self.widget.Inputs.data, table)
+        self.widget.row_check.setChecked(True)