By default, plt makes an X11 screen plot. To make a printed
plot, use the option -T lw, and pipe the output of plt to lwcat(1). Under
Unix, GNU/Linux, or MacOS/X, lwcat uses the standard lpr print spooler
to send plt's output in PostScript format to the default printer. When running
with a Cygwin/bash window under MS-Windows, or when using lwcat's -gv option
under Unix or Linux, the PostScript output is displayed on-screen using
GhostScript (GSView under MS-Windows, or gv otherwise; these programs can
save the output in a file or send it to a printer).