cot(x)=1/tan(x)
x(u,v)=D*(A*sin(b)*cos(u)+R*cos(v+phi)\
        *cos(u+W)-R*sin(m)*sin(v+phi)*sin(u+W))*exp(u*cot(a))
y(u,v)=(-A*sin(b)*sin(u)-R*cos(v+phi)\
        *sin(u+W)-R*sin(m)*sin(v+phi)*cos(u+W))*exp(u*cot(a))
z(u,v)=(-A*cos(b)+R*sin(v+phi)*cos(m))*exp(u*cot(a))
D=1;a=4*pi/9;b=2*pi/9;phi=11*pi/36;m=pi/18;W=pi/6;A=25;R=16
#
L=300
set xrange[-L+60:L+60]
set yrange[-L:L]
set zrange[-1.5*L:0.5*L]
set pm3d depth hidden3d 4  # means lt 4
set palette rgb 8,7,27
unset colorbox
set parametric
set ticslevel 0
set view 65,30,2.4,1
set vrange[0:2*pi]
unset key
unset border
unset tics
set isosamples 96,24
set urange[-2*pi:4*pi]
splot x(u,v),y(u,v),z(u,v) w pm3d 
pause 3
set term pngcairo size 480,480
set out "vs_natalina.png"
replot