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
* 
	 * @generated
	 * @ordered
	 */
	int TOOLTIP_VALUE__TEXT = ACTION_VALUE_FEATURE_COUNT + 0;
	/**
	 * The feature id for the '<em><b>Delay</b></em>' attribute. <!--
	 * begin-user-doc --> <!-- end-user-doc -->
	 * 
	 * @generated
	 * @ordered
	 */
	int TOOLTIP_VALUE__DELAY = ACTION_VALUE_FEATURE_COUNT + 1;
	/**
	 * The number of structural features of the the '<em>Tooltip Value</em>' class.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	int TOOLTIP_VALUE_FEATURE_COUNT = ACTION_VALUE_FEATURE_COUNT + 2;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.impl.URLValueImpl <em>URL Value</em>}' class.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.impl.URLValueImpl
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getURLValue()
	 * @generated
	 */
	int URL_VALUE = 35;
	/**
	 * The feature id for the '<em><b>Base Url</b></em>' attribute. <!--