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 the containment reference '{@link org.eclipse.birt.chart.model.attribute.Style#getColor <em>Color</em>}'.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return the meta object for the containment reference '<em>Color</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.Style#getColor()
	 * @see #getStyle()
	 * @generated
	 */
	EReference getStyle_Color( );
	/**
	 * Returns the meta object for the containment reference '{@link org.eclipse.birt.chart.model.attribute.Style#getBackgroundColor <em>Background Color</em>}'.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return the meta object for the containment reference '<em>Background Color</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.Style#getBackgroundColor()
	 * @see #getStyle()
	 * @generated
	 */
	EReference getStyle_BackgroundColor( );
	/**
	 * Returns the meta object for the containment reference '{@link org.eclipse.birt.chart.model.attribute.Style#getBackgroundImage <em>Background Image</em>}'.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return the meta object for the containment reference '<em>Background Image</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.Style#getBackgroundImage()
	 * @see #getStyle()
	 * @generated
	 */
	EReference getStyle_BackgroundImage( );
	/**