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.AngleType <em>Angle Type Object</em>}'.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Angle Type Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.AngleType
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.AngleType"
	 *        extendedMetaData="name='AngleType:Object' baseType='AngleType'" 
	 * @generated
	 */
	EDataType getAngleTypeObject( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.AxisType <em>Axis Type Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Axis Type Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.AxisType
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.AxisType"
	 *        extendedMetaData="name='AxisType:Object' baseType='AxisType'" 
	 * @generated
	 */
	EDataType getAxisTypeObject( );
	/**
	 * Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.ChartDimension <em>Chart Dimension Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Chart Dimension Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.ChartDimension
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.ChartDimension"
	 *        extendedMetaData="name='ChartDimension:Object' baseType='ChartDimension'" 
	 * @generated
	 */
	EDataType getChartDimensionObject( );