code: func NewHandlerFromGraphDriver(init graphDriver.InitFunc) *graphPlugin.Handler {
  	return graphPlugin.NewHandler(&shimDriver{driver: nil, init: init})
  }
docs: // NewHandlerFromGraphDriver creates a plugin handler from an existing graph
  // driver. This could be used, for instance, by the `overlayfs` graph driver
  // built-in to Docker Engine and it would create a plugin from it that maps
  // plugin API calls directly to any volume driver that satifies the
  // graphdriver.Driver interface from Docker Engine.