Ciao Luciano,
I can't test with actual serial input at this time but I believe that 
it's just a problem of refresh in the output window.

Have you tried to flush the window buffer after the plot statement?

...
plot x0% + (x%-1), y0% + (200-yold%) to x0%+ x%, y0% + (200-ynew% )
FLUSHWINDOWBUFFER
next x%

Michele.

luciano bernardi ha scritto:
> ...
> The problem I have now is to be able to “continuously” display the 
> data coming from an external device. The sample program I have 
> attached (adapted from a larger one) works well and reliably, but it 
> displays the data ONLY when all the 400 data had been received 
> (whereas I need to see them continuously, one after the other).
> ...
> Can someone give me a suggestion?
> Many thanks
> Luciano Bernardi