Adding shadow to a curve or some other objects can makes a 2d plot looks like a 3d one. The common way producing a shadow effect is drawing the object two times. Now let us come to an example of shadowing to a curve.
reset
set term png
set output "shadow1.png"
dy=0.75
dx=dy*tan(pi/6)
f(x)=0.1*(x10)*x*(x+10)
set xrange [15:15]
plot f(xdx)dy w l lw 7 lc rgb"#cccccc" notitle,\
f(x) w l lw 7 lc rgb"red" notitle

Simple shadow effect 
Firstly the shadow offset along yaxis and xaxis are given. The earlier plot is used to produce the shadow effcet and the later one is the data plotting. Note that the data plotting must be put after the shadow plotting, otherwise the "shadow" may cloud the datacurve. From the picture we find that the shadow effect have been realized, but it seems that the shadow is too sharp. We need to improve the script to get a better effect. Now let us do this. Our main idea is plotting the shadow sevral times with diffrent scale of gray colors. We realize this idea using a "for" iteration. The following is our improved script and the corresponding graph.
reset
set term png
set output "shadow2.png"
dy=0.75
dx=dy*tan(pi/6)
levels=15
f(x)=0.1*(x10)*x*(x+10)
set palette gray
set cbrange [0:1]
unset colorbox
set xrange [15:15]
set yrange [200:200]
set multiplot
plot for [i=levels:1:1] f(xi*(1.0/levels)*dx)i*(1.0/levels)*dy w l lw 7 \
lc palette cb i*(0.5/levels)+0.35 notitle
plot f(x) w l lw 7 lc rgb"red" notitle
unset multiplot

Improved shadow effect 
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