Sieve: Fix tests

Author: janezd

Orange/widgets/visualize/tests/test_owsieve.py

@@ -20,6 +20,7 @@ def setUp(self):
         self.widget = self.create_widget(OWSieveDiagram)
 
     def _select_data(self):
+        self.widget.attr_x, self.widget.attr_y = self.data.domain[:2]
         area = self.widget.areas[0]
         self.widget.select_area(area, QMouseEvent(
             QEvent.MouseButtonPress, QPoint(), Qt.LeftButton,