Bug 104986 Backgound image uses an absolute path
Currently Chart uses an absolute path to indicate a backgournd image, that may cause the design unusable on other environments. Expected the background image can be definded in multiple ways like in report designer. e.g. URI(absolute path), File(relative path), Embedded Image(Encoded Image Data).
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/render/AxesRenderer.java
}
	/**
	 * This method renders the bar graphic elements superimposed over the plot
	 * background and any previously rendered series' graphic elements.
	 */
	public final void renderPlot( IPrimitiveRenderer ipr, Plot p )
			throws ChartException
	{
		if ( !p.isVisible( ) ) 
		{
			return;
		}
		final ClipRenderEvent cre = (ClipRenderEvent) ( (EventObjectCache) getDevice( ) ).getEventObject( StructureSource.createPlot( p ),
				ClipRenderEvent.class );
		final boolean bFirstInSequence = ( iSeriesIndex == 0 );
		final boolean bLastInSequence = ( iSeriesIndex == iSeriesCount - 1 );
		final PlotWithAxes pwa = (PlotWithAxes) getComputations( );
		if ( bFirstInSequence )
		{
			renderBackground( ipr, p );
			renderAxesStructure( ipr, p );
		}
		ISeriesRenderingHints srh = null;
		try
		{
			srh = ( (PlotWithAxes) getComputations( ) ).getSeriesRenderingHints( getSeriesDefinition( ),
					getSeries( ) );
		}
		catch ( Exception ex )
		{