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
* A convenient method used in building the ticks for a datetime scale.
	 * Computes a new datetime object relative to the existing one moving back
	 * by 'step' units.
	 * 
	 * @param iUnit
	 * @param iStep
	 * 
	 * @return
	 */
	public CDateTime backward( int iUnit, int iStep )
	{
		CDateTime cd = (CDateTime) clone( );
		cd.add( iUnit, -iStep );
		return cd;
	}
	/**
	 * A convenient method used in building the ticks for a datetime scale.
	 * Computes a new datetime object relative to the existing one moving
	 * forward by 'step' units.
	 * 
	 * @param iUnit
	 * @param iStep
	 * @return
	 */
	public CDateTime forward( int iUnit, int iStep )
	{
		CDateTime cd = (CDateTime) clone( );
		cd.add( iUnit, iStep );
		return cd;
	}