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.RiserType <em>Riser Type Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Riser Type Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.RiserType
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.RiserType"
	 *        extendedMetaData="name='RiserType:Object' baseType='RiserType'" 
	 * @generated
	 */
	EDataType getRiserTypeObject( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.RuleType <em>Rule Type Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Rule Type Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.RuleType
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.RuleType"
	 *        extendedMetaData="name='RuleType:Object' baseType='RuleType'" 
	 * @generated
	 */
	EDataType getRuleTypeObject( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.ScaleUnitType <em>Scale Unit Type Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Scale Unit Type Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.ScaleUnitType
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.ScaleUnitType"
	 *        extendedMetaData="name='ScaleUnitType:Object' baseType='ScaleUnitType'" 
	 * @generated
	 */
	EDataType getScaleUnitTypeObject( );
	/**