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
return null; 
	}
	/**
	 * Extracts the group's key value that remains unchanged for a given group
	 * 
	 * @param iGroupIndex
	 *            The group index for which the key is requested
	 * @param iColumnIndex
	 *            The column index from which the group key value is to be
	 *            extracted
	 * 
	 * @return The group key value associated with the requested group index
	 */
	public Object getGroupKey( int iGroupIndex, int iColumnIndex )
	{
		final int iRow = ( iGroupIndex <= 0 ) ? 0
				: iaGroupBreaks[iGroupIndex - 1];
		if ( iRow >= 0 && iRow < workingResultSet.size( ) )
		{
			return ( (Object[]) workingResultSet.get( iRow ) )[iColumnIndex];
		}
		return null; 
	}
	/**
	 * Creates an instance of a resultset subset that uses references to
	 * dynamically compute a subset of the original resultset instance rather
	 * than duplicate a copy of the original resultset data content
	 * 
	 * @param iGroupIndex
	 *            The group number for which a subset is requested