Multiple calls to draw-line

I have a display-callback that draws a grayscale sonogram from fft data. To do 
this I'm making a lot of calls to draw-line, since (obviously) I need to draw 
each line, representing a frequency bin, with a different grayscale value 
(hence I can't use draw-lines or draw-polygon). This seems to slow the code up 
considerably. Here is the core loop within the function:

(let ((fft/2 (/ *fft-size* 2))
      (bin-width (round (/ %scroll-height% (/ *sample-rate* *fft-size*))))
      (delta (round (/ %scroll-width% *n-frames*))))
  (loop for k of-type fixnum being the hash-key of *spectrum*
        for fr of-type (simple-array double-float (*)) = (gethash k *spectrum*)
	for time of-type fixnum = (* delta k)
        do		  
	(loop for i of-type fixnum from 0 below fft/2
	      for freq = (the fixnum (rescale (* i bin-width) 
                                              (* bin-width fft/2) 
                                              %scroll-height%)) 
              for mag = (the double-float (aref fr (- fft/2 1 i)))
	      when (>= mag *cut-off*)
	      do
	      (gp:draw-line pixmap 
			    time freq (+ time delta) freq
		            :thickness bin-width
			    :foreground (make-gray (rescale (float mag 0.0s0)
                                                            *cut-off* 
                                                            1))))))

I have of course profiled, and whilst draw-line didn't seem to be on top of the 
stack a lot I still feel it's the culprit. I can't see a way to optimize this 
any more, so is there a different way I could do this, without having to make 
so many calls to draw-line?

Help/suggestions greatly appreciated,
Chris