Bug 188029 Top face is missing in Area chart of 2D+ with negative values
Top face is missing in Area chart of 2D+ with negative values. Please see attached screenshot.
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/computation/withaxes/StackedSeriesLookup.java
sg.alUnitPositions = new ArrayList( 8 );
		}
		
		if ( sg.alUnitPositions.size( ) <= iUnitIndex )
		{
			sg.alUnitPositions.add( new AxisSubUnit( ) );
		}
		return (AxisSubUnit) sg.alUnitPositions.get( iUnitIndex );
	}
	/**
	 * Returns an AxisUnit needed to 'remember' the position of the next stacked
	 * bar to be rendered. If a series is not 'stackable' or not 'set as
	 * stacked', this method will return 'null'.
	 * 
	 * @param ax
	 * @param se
	 * @param iUnitIndex
	 * 
	 * @return
	 */
	public final AxisSubUnit getUnit( Series se, int iUnitIndex )
	{
		
		StackGroup sg = (StackGroup) htSeriesToStackGroup.get( se );
		if ( sg == null )
		{
			return null;
		}
		
		if ( sg.alUnitPositions == null )