Merge pull request #1882 from mstrazar/heatmap_limit_low_high

OWHeatmap: consistent threshold limits and attribute checks
(cherry picked from commit e6cdc76)

 Conflicts:
	Orange/widgets/visualize/owheatmap.py

Orange/widgets/visualize/owheatmap.py

@@ -478,6 +478,9 @@ class Error(widget.OWWidget.Error):
         no_continuous = Msg("No continuous feature columns")
         not_enough_memory = Msg("Not enough memory to show this data")
 
+    class Warning(widget.OWWidget.Warning):
+        threshold_error = Msg("Low slider should be less than High")
+
     def __init__(self):
         super().__init__()
 
@@ -706,6 +709,13 @@ def set_dataset(self, data=None):
                 self.Information.sparse_densified()
 
         input_data = data
+
+        # Data contains no attributes or meta attributes only
+        if data is not None and len(data.domain.attributes) == 0:
+            self.Error.no_continuous()
+            input_data = data = None
+
+        # Data contains some discrete attributes which must be filtered
         if data is not None and \
                 any(var.is_discrete for var in data.domain.attributes):
             ndisc = sum(var.is_discrete for var in data.domain.attributes)
@@ -1396,7 +1406,15 @@ def update_grid_spacing(self):
             layout.setSpacing(self.SpaceX)
             self.__fixup_grid_layout()
 
+    def check_threshold_consistency(self):
+        return self.threshold_low < self.threshold_high
+
     def update_color_schema(self):
+        if not self.check_threshold_consistency():
+            self.Warning.threshold_error()
+            return
+        else:
+            self.Warning.clear()
         palette = self.color_palette()
         for heatmap in self.heatmap_widgets():
             heatmap.set_color_table(palette)