This is the official manual for the latest Org-mode release.
prologue header argument is for appending to the top of the code
block for execution. For example, a clear or reset code at the start of new
execution of a ‘src’ code block. A
reset for ‘gnuplot’:
:prologue "reset". See also epilogue.
(add-to-list 'org-babel-default-header-args:gnuplot '((:prologue . "reset")))