transparency in gnuplot

There is options to set the fill color in gnuplot to be transparent. But some friends of mine ofen complained that it never works. This is because to let the transparency really work, you also need to choose a transparency supported terminal type. For raster graphics "png" may be a choice, and for vector graphics it may be "pdf". You can use command "help transparent" to see the details.
There is an example.

reset
f(v,T)=4/sqrt(pi*T**3)*exp(-v**2/T)*v**2
set sample 500
set title "Maxwell speed distribution"
set xlabel "Speed [(2kT_c/m)^{1/2}]"
set ylabel "Probability Intensity f(v)"
set style fill transparent solid 0.5
set key Left
set term png truecolor enhanced font "Times,15"
set output "maxwell_speed_distribution.png"
plot [0:5] f(x,0.1) w filledcurves x1 title "T=0.1T_c",\
           f(x,1)   w filledcurves x1 title "T=T_c",\
           f(x,5)   w filledcurves x1 title "T=5T_c"
set term pdf enhanced
set output "maxwell_speed_distribution.pdf"
replot

Note that when transparency is used in png terminal, the "truecolor" option must be selected.

The png file is shown below and the pdf file can be downloaded here.