2d-Plot with symbols

[Nicolas Schneider <nicolas.schneider@xxxxxxxxxxxxx>]

Dear ferret users

I have 2-dimensional (x-y) data of temperature differences. I would now 
like to draw a 2D-plot with symbols instead of filled grid boxes 
(SHADE). That means that instead of a filled grid box, I would like to 
draw a symbol reflecting the value in that grid box. Positive 
temperature differences should be drawn with filled circles with the 
size depending on the magnitude of the temperature difference. 
Contrastingly, negative temperature differences should be drawn with 
open circles, again depending on the magnitude of the difference.

Is this possible with ferret? Where should I look for? Under SHADE 
commands or PLOT commands?

Thanks for any help,
Nicolas