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. 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).