Scrolling pixmaps

I'm drawing a lot of data using an off-screen pixmap (stored in a slot in my
main interface). The data typically consists of a large number of points, which
I draw to the pixmap using draw-polygon. So I need to scroll the pane, but it's
still rather laboured, and also flashes at the edges where the new data is
coming into view. I was under the impression that using an off-screen buffer
would eradicate problems like these. Here is some test code which illustrates
what I'm doing -

(define-interface pixmap-test ()
  ((pixmap :initform nil))
  (:panes
    (pixmap-object drawn-pinboard-object
      :display-callback 'draw-my-pixmap-object
      :internal-min-width 5000))
  (:layouts
    (pixmap-layout pinboard-layout
      '((pixmap-object))
	 :scroll-width 5000
	 :horizontal-scroll t))
  (:default-initargs
    :layout 'pixmap-layout
    :destroy-callback 'destroy-my-pixmap-object
    :best-width 500
    :best-height 500))

(defmethod interface-display :after ((self pixmap-test))
  (with-slots (pixmap pixmap-layout) self
    (with-geometry pixmap-layout
      (setf pixmap (gp:create-pixmap-port pixmap-layout %scroll-width% %height%
	              :clear t :background :white)))))

(defun destroy-my-pixmap-object (interface)
  (with-slots (pixmap) interface
    (when pixmap
      (gp:destroy-pixmap-port pixmap))))

(defun draw-my-pixmap-object (pane self x y width height)
  (declare (ignore self width height))
  (with-geometry pane
    (let ((coords 
            (loop for x-val from 0 to 3565
		  for x-coord = (truncate (rescale x-val 3565 %scroll-width%))
		  for y-val = (random 65536)
		  for y-coord = (truncate (rescale y-val 65536 %height%))
		  append (list x-coord y-coord))))
      (with-slots (pixmap) (element-interface pane)
        (gp:clear-graphics-port pixmap)
	(gp:draw-polygon pixmap coords :closed nil :foreground :blue)
	(gp:pixblt pane boole-1 pixmap x y %scroll-width% %height% x y)))))

The rescale function has the signature (rescale val old-max new-max), where both
intervals have a minimum of zero.

If I don't call clear-graphics-port then as I scroll the newly uncovered area
seems to get redrawn over the old one.

So what is actually happening here? Am I doing this right? I am wondering
whether, despite the fact that I'm using an off-screen buffer, scrolling causes
some redrawing to happen automatically behind the scenes.

Thanks for any advice, tips, etc.
Chris