The set psdir
directory
command controls the search path used by the
postscript terminal to find prologue.ps and character encoding files.
You can use this mechanism to switch between different sets of
locally-customized prolog files.
The search order is
1) The directory specified by `set psdir`, if any
2) The directory specified by environmental variable GNUPLOT_PS_DIR
3) A built-in header or one from the default system directory
4) Directories set by `set loadpath`