animated gif sequences

[Stephen Nash <s.d.nash@xxxxxxxxxxxxx>]

I am currently producing a sequnce of gif images in the -gif mode in order
to create a movie.


set window/aspect=2.5

define viewport/xlimits=0.000,1.000/ylim=0.667,1.000 upper 
define viewport/xlimits=0.000,1.000/ylim=0.333,0.667 middle 
define viewport/xlimits=0.000,1.000/ylim=0.000,0.333 lower

repeat/l=45:80 (set viewport upper;plot/vlimits=-4:4:1 divy;set viewport
middle;plot/v
limits=-4:4:1 divyww;set viewport lower;plot/vlimits=-4:4:1
divysw;frame/file=meridion
al_divergence.gif)


The first image is fine but each subsequent image just adds an overlay of
the latest frame.  Hence by the final frame I have a huge jumble of lines! 
How can I clear the window in -gif mode before starting on the next frame? 
The problem does not seem to occur either when producing maps as movies or
single plots - just when doing line plots in viewports.

Thank you,

Stephen