Bug 161219 Gantt chart with null datapoints fail to render
Create a dataset with two date columns, having some null date values Create a gantt chart Assign the dates to the gant series It fails to render and throw exception. Expected: null dates should be treated as: - 1 null date, 1 valid date ->one marker drawn, no line - 2 null dates -> skip rendering of datapoint If the decoration label is missing it should also not fail.
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/util/CDateTime.java
* 
	 * @param cdta
	 *            A datetime array for which the least significant unit
	 *            difference is to be computed
	 * 
	 * @return The least significant unit for which a difference in datetime
	 *         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] ) )
				{