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
{
				return cdt1.getYear( ) - cdt2.getYear( );
			}
			else
			{
				
				final double dDays = computeDifference( cdt1,
						cdt2,
						Calendar.DATE );
				return dDays / 365.25;
			}
		}
		return 0;
	}
	/**
	 * Walks through all values in a dataset and computes the least significant
	 * unit for which a difference was noted.
	 * 
	 * @param dsi
	 *            The dataset iterator that facilitates visiting individual
	 *            values
	 * 
	 * @return The least significant unit for which a difference in datetime
	 *         values was noted
	 */
	public static final int computeUnit( DataSetIterator dsi )
	{
		Calendar cCurr, cPrev;
		for ( int k = 0; k < iaUnitTypes.length; k++ )
		{