bug fix with the history graph plotting (an invalid point was added to the graph)

src/serverlogging.cpp

@@ -297,9 +297,13 @@ void CHistoryGraph::Update()
 
         // add markers
         for ( i = 0; i < iNumItemsForHistory; i++ )
+        {
+            // only use valid dates
+            if ( vHistoryDataFifo[i].DateTime.date().isValid() )
             {
                 AddMarker ( vHistoryDataFifo[i] );
             }
+        }
 
         // save graph as picture in file
         Save ( sFileName );