Bug 202254 Exception is thrown when category values are null and type of X axis is set to DateTime
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/util/CDateTime.java
*         values was noted
	 */
	public static final int computeUnit( CDateTime[] cdta )
			throws ChartException
	{
		int j;
		for ( int k = 0; k < iaUnitTypes.length; k++ )
		{
			for ( int i = 0; i < cdta.length; i++ )
			{
				j = i + 1;
				if ( j > cdta.length - 1 )
				{
					j = 0;
				}
				if ( cdta[i] == null || cdta[j] == null )
				{
					throw new ChartException( ChartEnginePlugin.ID,
							ChartException.VALIDATION,
							"exception.base.orthogonal.null.datadefinition", 
							Messages.getResourceBundle( ) );
				}
				if ( cdta[i].get( iaUnitTypes[k] ) != cdta[j].get( iaUnitTypes[k] ) )
				{
					return iaUnitTypes[k]; 
					
				}
			}
		}