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
}
		else if ( iUnit == Calendar.WEEK_OF_YEAR )
		{
			final double dDays = computeDifference( cdt1, cdt2, Calendar.DATE );
			double rt = dDays / 7.0;
			if ( trimBelow )
			{
				rt = Math.floor( rt );
			}
			return rt;
		}
		else if ( iUnit == Calendar.MONTH )
		{
			if ( trimBelow )
			{
				final double dYears = cdt1.getYear( ) - cdt2.getYear( );
				return dYears * 12 + ( cdt1.getMonth( ) - cdt2.getMonth( ) );
			}
			else
			{
				
				final double dDays = computeDifference( cdt1,
						cdt2,
						Calendar.DATE );
				return dDays / 30.4375;
			}
		}
		else if ( iUnit == Calendar.YEAR )
		{
			if ( trimBelow )