Bug 161222 Decoration label should be optional in Gantt Chart
It should not be mandatory to specify a decoration label for Gantt Charts, this can be left blank. Currently the UI and Engine enforce a data definition to be specified for it.
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/internal/datafeed/ResultSetWrapper.java
* 
	 * @param iColumnIndex
	 * @return an array have two return objects, first is the base value list,
	 *         second is the index map for all grouped subset.
	 */
	public Object[] getMergedGroupingBaseValues( int iColumnIndex,
			SortOption sorting )
	{
		int groupCount = getGroupCount( );
		final List idxList = new ArrayList( groupCount );
		Object oValue;
		ResultSetDataSet rsd;
		final List baseValue = new ArrayList( );
		List idx;
		for ( int k = 0; k < groupCount; k++ )
		{
			rsd = getSubset( k, iColumnIndex );
			idx = new ArrayList( );
			if ( k == 0 )
			{
				
				int i = 0;
				while ( rsd.hasNext( ) )
				{
					oValue = rsd.next( )[0];
					baseValue.add( oValue );
					idx.add( new Integer( i++ ) );
				}
			}
			else