14.8.2.26 :prologue

The value of the prologue header argument will be prepended to the code block body before execution. For example, :prologue "reset" may be used to reset a gnuplot session before execution of a particular code block, or the following configuration may be used to do this for all gnuplot code blocks. Also see epilogue.

(add-to-list 'org-babel-default-header-args:gnuplot
             '((:prologue . "reset")))