Transparency in plotted surfaces

Question

I'm using Octave 7.2.0-1 on linux.

When plotting surfaces which overlap (both with facealpha<1) Octave only shows the axes as a "see-through", not the surface which is "behind":

[ xs, ys, zs ] = sphere( 20 );
surf( xs, ys, zs, 'facealpha', 0.5, 'edgealpha', 0.5, 'facecolor', 'r' );
hold on
surf( xs+1, ys+1, zs , 'facealpha',0.5, 'edgealpha', 0.5, 'facecolor', 'g');

Is this a bug?