Fltk toolkit, grid is shown *beneath* pcolor (2D density plot)

[h6mgos8bn6fb]

Hello!

Octave version 4.0.3 (Debian Stretch; sorry, didn't check latest version).

x=linspace(-1.1,1.1,100);
y=linspace(-1.1,1.1,100);
[xx,yy]=meshgrid(x,y);
zz=sinc(sqrt(xx.^2+yy.^2));
pcolor(xx,yy,zz);
shading('interp')
grid on

Result -- grid is shown *beneath* pcolor, so pretty useless:
http://img12.lostpic.net/2018/02/08/c16462508e969acf488fc0d463c87aaa.png

I tried to put grid immediately after pcolor -- not working either. With 
gnuplot works fine.

Thanks.