bnc#590020: Sort data points stably.

std::sort is unstable sort, which is not what we want.  Use std::stable_sort
to sort data points by X values.  If we use unstable sort, it may mess up
the order of the sequence when two data points contain identical X values.

Change-Id: I6453a986185b326dc680fbcec6227ea332235b22

chart2/source/view/main/VDataSeries.cxx

@@ -302,7 +302,7 @@ void VDataSeries::doSortByXValues()
         }
 
         //do sort
-        std::sort( aTmp.begin(), aTmp.end(), lcl_LessXOfPoint() );
+        std::stable_sort( aTmp.begin(), aTmp.end(), lcl_LessXOfPoint() );
 
         //fill the sorted points back to the members
         m_aValues_X.Doubles.realloc( m_nPointCount );