问如何制作透明的作图点

EN

### attempt to mimic a radar screen
reset session

# create some test landscape
set xrange [-8:8]
set yrange [-8:8]
set samples 50 
set isosamples 50
set contour
set cntrparam level 5
set table $Data 
    splot (sin(1.3*x)*cos(.9*y)+cos(.8*x)*sin(1.9*y)+cos(y*.2*x))*3
unset table
unset contour
set angle degrees
Angle(dx,dy) = dx==dx && dy==dy ? dx==0 ? dy==0 ? NaN : \
               abs(sgn(dy))*180-sgn(dy)*90 : dx<0 ? 180+atan(dy/dx) : \
               dy<0 ? atan(dy/dx)+360 : atan(dy/dx) : NaN
r(x,y) = sqrt(x**2 + y**2)
set table $Contour
    plot $Data u (Angle($1,$2)):(r($1,$2)) index 1::1 
unset table

set size ratio 1
set polar
unset border
set border polar lc "green"
unset xtics
unset ytics
set format r ""
unset raxis
set rtics scale 0
set format t ""
set grid lw 2 lc "green"
set obj 1 rect from screen 0,0 to screen 1,1 fc "black" behind     # black background

stop = 0
bind x "stop=1"

$Data <<EOD
45    5
0   4
123   3.5
325   3.5
EOD

unset key
set trange [0:360]
set rrange [0:8]

alpha(a,a0)    = int((1-exp(-real(a)/a0))*255)<<24       # transparency "decay" 
myBaseColor    = 0x00ff00
myColorA(a)    = myBaseColor + alpha(a,45)               # decay for radar "beam"
AngleDiff(a,b) = b>a ? b-a : 360-a+b
myColorB(a,b)  = myBaseColor + alpha(AngleDiff(a,b),220) # decay for data

a=90; step=5
while (!stop) {
    a = a<step ? a=a+360-step : a-step
    do for [i=0:180] {
        set obj 100+i circle at 0,0 size 8 arc [a+i:a+i+2] fs solid noborder fc rgb myColorA(i) behind
    }
    plot $Data u 1:2:(myColorB(a,$1)) w p pt 7 lc rgb var, \
         $Contour u 1:2:(myColorB(a,$1)) w l lc rgb var
}
### end of code