x(u,v)= v < pi ? (2.5-1.5*cos(v))*cos(u): \
        v < 2*pi ? (2.5-1.5*cos(v))*cos(u):\
        v < 3*pi ? -2+(2+cos(u))*cos(v): -2+2*cos(v)-cos(u)
y(u,v)= v < pi ? (2.5-1.5*cos(v))*sin(u): \
        v < 2*pi ? (2.5-1.5*cos(v))*sin(u): \
        v < 3*pi ? sin(u): sin(u)
z(u,v)= v < pi ? -2.5*sin(v): v < 2*pi ? 3*v-3*pi:\
        v < 3*pi ? (2+cos(u))*sin(v)+3*pi: -3*v+12*pi 
#
set style line 100 lw 0.1 lc rgb "white"
set pm3d depthorder hidden3d ls 100
set parametric
set pal defined (0 "olive", 1 "dark-olivegreen" )
unset colorbox
unset key
unset border
unset tics
set ticslevel 0
set view 70,210,1,1.4
set isosamples 18,48
set urange[0:2*pi]
set vrange[0:4*pi]
splot x(u,v),y(u,v),-z(u,v) w pm3d
pause 3
set term pngcairo size 480,480
set out "vessel.png"
replot