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 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 loadpath