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 class '{@link org.eclipse.birt.chart.model.attribute.CallBackValue <em>Call Back Value</em>}'.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return the meta object for class '<em>Call Back Value</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.CallBackValue
	 * @generated
	 */
	EClass getCallBackValue( );
	/**
	 * Returns the meta object for the attribute '{@link org.eclipse.birt.chart.model.attribute.CallBackValue#getIdentifier <em>Identifier</em>}'.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return the meta object for the attribute '<em>Identifier</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.CallBackValue#getIdentifier()
	 * @see #getCallBackValue()
	 * @generated
	 */
	EAttribute getCallBackValue_Identifier( );
	/**
	 * Returns the meta object for class '{@link org.eclipse.birt.chart.model.attribute.ColorDefinition <em>Color Definition</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for class '<em>Color Definition</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.ColorDefinition
	 * @generated
	 */
	EClass getColorDefinition( );
	/**
	 * Returns the meta object for the attribute '{@link org.eclipse.birt.chart.model.attribute.ColorDefinition#getTransparency <em>Transparency</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for the attribute '<em>Transparency</em>'.