Bug 317267 [Automation][Regression]Error is thrown out when preview gantt type chart
chart/org.eclipse.birt.chart.engine.extension/src/org/eclipse/birt/chart/extension/datafeed/GanttDataSetProcessorImpl.java
{
			for ( int i = 0; i < obja.length - 1; i++ )
			{
				if ( !( obja[i] instanceof CDateTime ) )
				{
					valid = false;
					break;
				}
			}
		}
		if ( !valid )
		{
			throw new ChartException( ChartEngineExtensionPlugin.ID,
					ChartException.VALIDATION,
					"exception.dataset.invalid.ganttentry", 
					Messages.getResourceBundle( getULocale( ) ) );
		}
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.chart.datafeed.IDataSetProcessor#getMinimum(org.eclipse.birt.chart.model.data.DataSet)
	 */
	public final Object getMinimum( DataSet ds ) throws ChartException
	{
		DataSetIterator dsi = null;
		try
		{
			dsi = new DataSetIterator( ds );
			dsi.reset( );