Bug 212370 [Regression] Negative value can not be shown both in chart builder and when preview [13]
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/computation/withaxes/AxisSubUnit.java
{
		dValueLastNegative = 0;
		dValueLast = 0;
		dLastMaxPosition = Double.NaN;
		dLastMinPosition = Double.NaN;
	}
	
	/**
	 * Returns if current positive and negative values are aggregated together
	 * or not
	 * 
	 * @return true: together, false: by sign respectively
	 */
	public boolean isStackTogether( )
	{
		return bStackTogether;
	}
	
	/**
	 * Accumulates the value and returns the result.
	 * 
	 * @param dValue
	 *            the value to accumulate
	 * @return the result value after accumulating
	 * @see #isStackTogether()
	 * @see #getStackedValue(double)
	 */
	public final double stackValue( double dValue )
	{
		if ( bStackTogether )