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
sg.getGroupingInterval( ),
					iafa );
		}
		else if ( dtGrouping == DataType.DATE_TIME_LITERAL )
		{
			groupDateTime( workingResultSet,
					iSortColumnIndex,
					saExpressionKeys,
					iaGroupBreaks,
					null,
					sg.getGroupingInterval( ),
					sg.getGroupingUnit( ),
					iafa );
		}
		else if ( dtGrouping == DataType.TEXT_LITERAL )
		{
			groupTextually( workingResultSet,
					iSortColumnIndex,
					saExpressionKeys,
					iaGroupBreaks,
					null,
					sg.getGroupingInterval( ),
					iafa );
		}
		
		
		initializeMeta( );
	}
	private void groupNumerically( List resultSet, int iBaseColumnIndex,
			String[] saExpressionKeys, int[] iaBreaks,