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
* @see org.eclipse.birt.chart.model.attribute.HorizontalAlignment
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.HorizontalAlignment"
	 *        extendedMetaData="name='HorizontalAlignment:Object' baseType='HorizontalAlignment'" 
	 * @generated
	 */
	EDataType getHorizontalAlignmentObject( );
	/**
	 * Returns the meta object for data type '{@link java.lang.String <em>ID</em>}'.
	 * <!-- begin-user-doc --> <!--
	 * end-user-doc -->
	 * @return the meta object for data type '<em>ID</em>'.
	 * @see java.lang.String
	 * @model instanceClass="java.lang.String"
	 *        extendedMetaData="name='ID' baseType='http://www.eclipse.org/emf/2003/XMLType#string' pattern='[A-Z]'" 
	 * @generated
	 */
	EDataType getID( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.IntersectionType <em>Intersection Type Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Intersection Type Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.IntersectionType
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.IntersectionType"
	 *        extendedMetaData="name='IntersectionType:Object' baseType='IntersectionType'" 
	 * @generated
	 */
	EDataType getIntersectionTypeObject( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.LeaderLineStyle <em>Leader Line Style Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->