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
private static final int MILLIS_IN_HOUR = MILLIS_IN_MINUTE * 60;
	private static final int MILLIS_IN_DAY = MILLIS_IN_HOUR * 24;
	private static final int[] iaUnitTypes = {
			Calendar.MILLISECOND,
			Calendar.SECOND,
			Calendar.MINUTE,
			Calendar.HOUR_OF_DAY,
			Calendar.DATE,
			Calendar.WEEK_OF_YEAR,
			Calendar.MONTH,
			Calendar.YEAR
	};
	private static int[] iaCalendarUnits = {
			Calendar.SECOND,
			Calendar.MINUTE,
			Calendar.HOUR_OF_DAY,
			Calendar.DATE,
			Calendar.MONTH,
	};
	private static final SimpleDateFormat _sdf = new SimpleDateFormat( "MM-dd-yyyy HH:mm:ss" ); 
	/**
	 * A zero-arg default constructor
	 */
	public CDateTime( )
	{
		super( );
	}
	/**
	 * A constructor that creates an instance from a given
	 * <code>java.util.Date</code> value