Macos – WARNING: Plotting with an ‘unknown’ terminal. No output will be generated. Please select a terminal with ‘set terminal’

i am following the great machine learning course given by Andrew Wu from Stanford. When I executed plot function in ex5 file, the octave cli command line reports the following warning:

set terminal aqua enhanced title "Figure 1" size 560 420  font "*,6" dashlength 1
                      ^
         line 0: unknown or ambiguous terminal type; type just 'set terminal' for a list

WARNING: Plotting with an 'unknown' terminal.
No output will be generated. Please select a terminal with 'set terminal'.

The calculation works but all the graphs were not drew by plot.
I tried set terminal qt or others like x11, but none of them worked.
The solution in this post Can't find x11 terminal in gnuplot Octave on Mac OS does not work either.

my mac is Yosemite 10.10.3
my gnuplot is

G N U P L O T
Version 5.0 patchlevel 1    last modified 2015-06-07 
...
Terminal type set to 'unknown'

my XQuartz 2.7.7 (xorg-server 1.15.2)

Anyone knows how to fix? Thanks!