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/CallBackValue.java
{
	/**
	 * Returns the value of the '<em><b>Identifier</b></em>' attribute.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * <!-- begin-model-doc -->
	 * Specify a call back identifier to be used in the call back routine.
	 * <!-- end-model-doc -->
	 * @return the value of the '<em>Identifier</em>' attribute.
	 * @see #setIdentifier(String)
	 * @see org.eclipse.birt.chart.model.attribute.AttributePackage#getCallBackValue_Identifier()
	 * @model unique="false" dataType="org.eclipse.emf.ecore.xml.type.String" required="true"
	 *        extendedMetaData="kind='element' name='Identifier'"
	 * @generated
	 */
	String getIdentifier( );
	/**
	 * Sets the value of the '{@link org.eclipse.birt.chart.model.attribute.CallBackValue#getIdentifier <em>Identifier</em>}' attribute.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @param value the new value of the '<em>Identifier</em>' attribute.
	 * @see #getIdentifier()
	 * @generated
	 */
	void setIdentifier( String value );
}