set size ratio -1
set xrange [0:300]
set yrange [0:300]
unset key
set parametric
set samples 1000
set urange [0:2*pi/3]
set label "{plot with rgbalpha}" at sc 0.4,0.15 \
    font "Helvetica,18" tc rgb "dark-green" front
set term pngcairo enh font "Times,10" size 480,360
set out "plot_rgbalpha.png"
plot \
150+150*sin(3*t), 150+150*cos(5*t) lw 3 lc rgb "dark-yellow",\
"lena_tr.png" binary filetype=png w rgbalpha