i was unable to do polygon  :-\

still don't quite understand the logic.

nevermind i try some other time.

i stick to analytic curve  :D
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The required properties for polygon are:
1. number of connected point (integer)
2. x,y coordinates for those n points (so two double array are required).
For drawing half of a circle (from 0-Math.PI).
You can define
n: 20 (integer) // set initial value to 20
R: 10. (double) // radius for circle
x[n]: (double) // it is not easy to set 20 values at the variable page in model panel, so we will do it in initialize page (or custom page).
y[n]: (double)
where x[n],y[n] are double array.

np=20;
double dc=Math.PI/n ,c; // define temperate variables:  dc,c
for(int i=0;i< n;i++){
c=i*dc;
x[i]=R*Math.cos(c);
y[i]=R*Math.sin(c);
}
In View page:
Assign x,y to X,Y properties for polygon, n to number of points.
And you will get a half circle drawing.
You can play with other properties to see what will be changed.
(You are a quick learner, you will find out by yourself).