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.Orientation <em>Orientation Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Orientation Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.Orientation
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.Orientation"
	 *        extendedMetaData="name='Orientation:Object' baseType='Orientation'" 
	 * @generated
	 */
	EDataType getOrientationObject( );
	/**
	 * Returns the meta object for data type '<em>Percentage</em>'. <!--
	 * begin-user-doc --> <!-- end-user-doc -->
	 * 
	 * @return the meta object for data type '<em>Percentage</em>'.
	 * @model instanceClass="double"
	 * @generated
	 */
	EDataType getPercentage( );
	/**
	 * Returns the meta object for data type '
	 * {@link java.lang.Double <em>Percentage Object</em>}'. <!--
	 * begin-user-doc --> <!-- end-user-doc -->
	 * 
	 * @return the meta object for data type '<em>Percentage Object</em>'.
	 * @see java.lang.Double
	 * @model instanceClass="java.lang.Double"
	 * @generated
	 */
	EDataType getPercentageObject( );
	/**