Octave and gnuplot can't plot on X11

Forrest Sheng Bao <http://forrest.bao.googlepages.com>

This morning I found that my Octave can't plot, like this

octave:1> plot(0:10,sin(0:10))
octave:2> Expected X11 driver: /usr/lib/gnuplot/gnuplot_x11
Exec failed: No such file or directory
See 'help x11' for more details

Since Octave actually called gnuplot to plot diagrams, I tried gnuplot

gnuplot> test

No pop-up X11 window but only output on the shell as ASCII text. Arrgh, how could this happen?

Then I tried a script programmed by myself

forrest@flavia:/forrest/work/ee/ad_hoc/wang/plots$ gnuplot plot3.gp
*** X11 output driver not found, switching to dumb terminal!
*** If you want to use the X11 output, please install the gnuplot-x11 package

Oh, I found the problem. Who removed my gnuplot-x11 package? So, I used apt-get to install it again and everything is OK.