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.MONTH )
		{
			return MILLIS_IN_DAY * 30.4375;
		}
		else if ( iUnit == Calendar.YEAR )
		{
			return MILLIS_IN_DAY * 365.25;
		}
		return 0;
	}
	/**
	 * Zeroes out all units for this datetime instance below a specified unit.
	 * 
	 * @param iUnit
	 *            The unit below which all values are to be zeroed out
	 */
	public final void clearBelow( int iUnit )
	{
		if ( iUnit == YEAR )
		{
			set( Calendar.MILLISECOND, 0 );
			set( Calendar.SECOND, 0 );
			set( Calendar.MINUTE, 0 );
			set( Calendar.HOUR, 0 );
			set( Calendar.DATE, 1 );
			set( Calendar.AM_PM, AM );
			set( Calendar.MONTH, 0 );
		}
		else if ( iUnit == MONTH )
		{