...but here it is so you don't have to scroll through all of that:
Build gnuplot with:
sudo make install
and if necessary change the gnuplot's path in octave with:
Just use the right mouse button to select the first point, then click a second time to select the other corner. Whala! Press "a" to go back to the original zoom level, and press "h" to dump some help out to the Octave terminal window.