Each PostScript output file includes a %%Prolog section and possibly some
additional user-defined sections containing, for example, character encodings.
These sections are copied from a set of PostScript prologue files that are
either compiled into the gnuplot executable or stored elsewhere on your
computer. A default directory where these files live is set at the time
gnuplot is built. However, you can override this default either by using the
gnuplot command set psdir or by defining an environment variable
GNUPLOT_PS_DIR. See set psdir (p. ).