The default gnuplot terminal in Mac OS X 10.8 used to pop up in aqua. In version 5.32.1 that no longer works. Instead the plot2d command fails quietly, with the only clue coming from trying to run the command:
plot2d( x^2, [x, -4, 4], [plot_format, gnuplot] );
set term wxt size 640,480 font ",12"; set term pop
"/Users/sean/maxout.gnuplot", line 1: unknown or ambiguous terminal type; type just 'set terminal' for a list
It appears that the default terminal on Mac OS X has been changed to wxt.