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/AttributeFactory.java
* <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return a new object of class '<em>Call Back Value</em>'.
	 * @generated
	 */
	CallBackValue createCallBackValue( );
	/**
	 * Returns a new object of class '<em>Color Definition</em>'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return a new object of class '<em>Color Definition</em>'.
	 * @generated
	 */
	ColorDefinition createColorDefinition( );
	/**
	 * Returns a new object of class '<em>Data Point</em>'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return a new object of class '<em>Data Point</em>'.
	 * @generated
	 */
	DataPoint createDataPoint( );
	/**
	 * Returns a new object of class '<em>Data Point Component</em>'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return a new object of class '<em>Data Point Component</em>'.
	 * @generated
	 */
	DataPointComponent createDataPointComponent( );
	/**
	 * Returns a new object of class '<em>Date Format Specifier</em>'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->