--- a/GraphicViewer.py	Wed Jun 06 19:02:30 2012 +0200
+++ b/GraphicViewer.py	Wed Jun 06 19:04:18 2012 +0200
@@ -224,6 +224,8 @@
         self.StartTick = 0
         self.StartIdx = 0
         self.EndIdx = 0
+        self.MinValue = None
+        self.MaxValue = None
         self.CursorIdx = None
         self.Fixed = False
         self.Ticktime = self.DataProducer.GetTicktime()