shading faceted / interp not working with trisurf ?

[lo]

hello,

using octave 3.6.4 on my suse 12.3 64 bits,
I want to plot a triangulated surface and use a smooth color interpolation;
but, as a simple example, the following

x = [ 0 1 2 0 1 2]';
y = [ 0 0 0 1 1 1]';
z = [ 0 0 0 0 0 0]';
tri = [ 1 2 4; 2 4 5; 2 3 5; 3 5 6];
c = [ 1 2 3 4 4 6 ]';
trisurf(tri,x,y,z,c);
shading interp;

does not produce what I expect;
it gives the same thing as
shading flat
shading faceted does not work either, i.e. same results as with shading 
flat;

is this piece of code correct ?

I am not quite sure that this is related to octave;
could depend on my gnuplot (4.6.3), X server or whatever involved for 
the rendering;

I would welcome any suggestion,
thanks in advance,

regards,
Laurent