When draw a lot of figures and close them (click the windows control- "x" button) , Octave does not end the corresponding "gnuplot" and "gnuplot_x11" process. As a result, after a series of drawing and close the figures, Octave end up with the following error:
Maximum number of clients reached gnuplot: unable to open display ':0'
gnuplot: X11 aborted. I
Command "ps aux | grep gnuplot" shows a lot of "gnuplot" and "gnuplot_x11" processes in interruptible sleep states ("S+")