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/model/attribute/AttributePackage.java
* Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.SortOption <em>Sort Option Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Sort Option Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.SortOption
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.SortOption"
	 *        extendedMetaData="name='SortOption:Object' baseType='SortOption'" 
	 * @generated
	 */
	EDataType getSortOptionObject( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.Stretch <em>Stretch Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Stretch Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.Stretch
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.Stretch"
	 *        extendedMetaData="name='Stretch:Object' baseType='Stretch'" 
	 * @generated
	 */
	EDataType getStretchObject( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.StyledComponent <em>Styled Component Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Styled Component Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.StyledComponent
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.StyledComponent"
	 *        extendedMetaData="name='StyledComponent:Object' baseType='StyledComponent'" 
	 * @generated
	 */
	EDataType getStyledComponentObject( );
	/**