How to put a rectangle in the key with same hue as a shaded area in Gnuplot?

I have some shaded area in a plot and I'd like to label that shaded area with a small rectangle of the same hue as the shading in the plot.

I'm using gnuplot - my shading in the plot is achieved with

plot  'XXXX.dat' u 1:($2+$3):($2-$3) w filledcurve ls 999 fs solid 0.2 notitle,\

and I'd like to label the shading area in the key with a small rectangle of the same hue.

Is it possible? I'm also happy to set it manually via

set object rectangle from x1,y1 to x2,y2 fc rgb "blue"

but in this way I've only managed to sample colours from the standard gnu plot palette and not the hue I've set for the shading via fs solid 0.2.

Thanks!

I'm not sure if I understand your problem. If you want the filled area in your key you have to define a keyentry via title but you are using notitle.

Code:

set key out
plot '+' u 1:($1**2) w filledcurves ls 999 fs solid 0.2 title "filled area"

Result: