prerm is in gnuplot-x11 4.6.6-3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8  | #!/bin/sh
set -e
if [ "$1" != "upgrade" ]; then
    update-alternatives --remove gnuplot /usr/bin/gnuplot4-x11
    update-alternatives --remove gnuplot4 /usr/bin/gnuplot4-x11
fi
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