set border linecolor rgb "gold"
set key title "Plot style rgbalpha" textcolor rgb "gold" 
set key bottom left at screen .75, screen .75
set key Left width -7 sample 1

set object  1 rect from screen 0, 0 to screen 1, 1 behind \
              fc rgb "gray10"  fillstyle  solid 1.00 noborder
set samples 128, 128
set size ratio 0.95
set bmargin at screen .1
set tmargin at screen .9
set lmargin at screen .1


set xrange [ 0. : 128. ] 
set yrange [ 0. : 128. ]

# Alpha =  linear gradient on x

plot 100.*(.4+sin(x/5.)/(x/5.)) lw 5 title 'solid line', \
     'lena.rgb' binary array=(128,128) format="%uchar" flipy using 1:2:3:(2.*column(0)) \
                with rgbalpha title "Lena with linear\nalpha gradient"


Click here for minimal script to generate this plot