Bug 207268 Add visibility option for plot within axes
For chart with axes, we should make plot within axes invisible. Meantime, other plots, such as axes, legend and title, should be visible. This enhancement doesn't include UI change, and it is required for cross tab integration.
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/render/AxesRenderer.java
return IConstants.EQUAL;
		if ( de1 == null || de2 == null )
			return IConstants.SOME_NULL;
		final Class c1 = de1.getClass( );
		final Class c2 = de2.getClass( );
		if ( c1.equals( c2 ) )
		{
			if ( de1 instanceof NumberDataElement )
			{
				return Double.compare( ( (NumberDataElement) de1 ).getValue( ),
						( (NumberDataElement) de2 ).getValue( ) );
			}
			else if ( de1 instanceof DateTimeDataElement )
			{
				final long l1 = ( (DateTimeDataElement) de1 ).getValue( );
				final long l2 = ( (DateTimeDataElement) de1 ).getValue( );
				return ( l1 < l1 ? IConstants.LESS : ( l1 == l2
						? IConstants.EQUAL : IConstants.MORE ) );
			}
			else if ( de1 instanceof TextDataElement )
			{
				throw new ChartException( ChartEnginePlugin.ID,
						ChartException.DATA_FORMAT,
						"exception.unsupported.compare.text",  
						Messages.getResourceBundle( getRunTimeContext( ).getULocale( ) ) );
			}
			else
			{
				throw new ChartException( ChartEnginePlugin.ID,
						ChartException.DATA_FORMAT,