I found that if I run octave in a terminal with dark background and light foreground, then the output of a plot command shows only the plot itself, not the axes and tic marks. The background of the plot is black, too. I think the reason is that the default colors of axes and tic marks are black, and they are indistinguishable from the black background. However, if I run gnuplot in the same terminal, the output is normal: light background and black axes. Since octave plot commands evoke gnuplot, I don't see where the problem is.