Do the following
cd /Applications/Gnuplot.app/Contents/Resources/lib # move into directory
mv libfreetype.6.dylib libfreetype.6.dylib.bak # make a backup of current library
ln -s /usr/X11/lib/libfreetype.6.dylib . # create a symbolic link to proper library
from: http://azaleasays.com/2012/10/20/octave-graphic-error-on-mac-os-x-mountain-lion/