Gnuplot: line opacity / transparency?

Good news! This has been implemented in gnuplot. Example syntax is

plot x lw 10, -x lw 10 lc rgb "#77000000"

This will plot x as a red line and -x as a transparent black line (it looks gray). The first pair of two characters in the rgb specification define the alpha (transparency) channel ("#AARRGGBB"). The normal syntax ("#RRGGBB") still works.

old (gnuplot < 5.0 or so) answer for reference:

If you want to make lines plotted for time series data, the answer is no (see discussion here). You can't set a line style to be transparent. Transparency only works for filling under curves, and it has to be printed to the right terminal type.

I ran into this problem myself recently, I hope this feature will be added in a future version of gnuplot.