octave plots/graphics

[Bruno Schneider <boschneider@gmail.com>]

I have just installed octave and it seems I'm missing some parts. I'm
using Debian testing with XFCE. I have the following packages
installed:

$ dpkg -l | grep octave
ii  liboctave6:amd64                      4.4.1-5
ii  octave                                4.4.1-5
ii  octave-bsltl                          1.1.1-2
ii  octave-common                         4.4.1-5
ii  octave-doc                            4.4.1-5
ii  octave-image                          2.10.0-2

On octave, i enter the following commands:

x = linspace(0, 2*pi);
a = cos(2*x);
plot(x, a);

Then, a window appears with menus, but the drawing is completely blank.

I get some font errors. I don't know if they affect the graph:

warning: ft_manager: unable to load font:
/usr/share/fonts/opentype/freefont/FreeSans.otf
warning: called from
    axes at line 66 column 10
    newplot at line 158 column 10
    plot at line 222 column 9
warning: ft_text_renderer: unable to load appropriate font

It seems I'm using Qt for graphics but my system seems to have all
libqt5* packages installed. I also tried gnuplot graphics and the
window was also blank.

Am I missing packages?

Can I configure octave to use other fonts instead?
-- 
Bruno Schneider