Bug 211839 String grouping unit type needs to support interval range.
Build ID: I20071101-2000 As Chart has supported, the String grouping unit type can be set interval range, the interval range means that the string group will base on row count distinction. For instance, let's say we have a,b,c,d,e,f,g , and set interval range to 2, the grouping result should be a,c,e,g. Now it is a requirement that DtE support the grouping type.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/executor/transform/group/GroupBy.java
groupBy.columnName = columnName;
		return groupBy;
	}
	/**
	 * Static method to create and instance of subclass of GroupBy, based on the
	 * group definition
	 * 
	 * @param groupDefn
	 * @return GroupBy
	 * @throws DataException
	 */
	public static GroupBy newInstanceForRowID( GroupSpec groupDefn )
			throws DataException
	{
		assert groupDefn != null;
		GroupBy groupBy = null;
		switch ( groupDefn.getInterval( ) )
		{
			case GroupSpec.NO_INTERVAL :
				groupBy = new GroupByDistinctValue( );
				break;
			case GroupSpec.NUMERIC_INTERVAL :
				groupBy = new GroupByPositionRange( groupDefn );
				break;
			default :
				throw new DataException( ResourceConstants.BAD_GROUP_INTERVAL_TYPE_ROWID );
		}
		groupBy.groupSpec = groupDefn;
		groupBy.columnIndex = -1;
		return groupBy;