I was going to say that the missing piece of information to be able to give you a relevant reply is: how is your polygon defined? There are lots of possibilities in many coordinate systems, but I bet you just have the x-y coordinates of the corners in the same coordinate system as your data points.

On that assumption, the very first post provides a very elegant method.

Here's how you might code it in Perl, in very basic style for clarity.

A further assumption is that you have already been told how to build the polygon from the corner coordinates - you know which dots are connected to which so you don't have to try every polygon possible for a given set of corner points.

#assume we start with the corner points ordered in order of drawing th
+e polygon
@x = (x1, x2, x3, ....);
@y = (y1, y2, y3, ....);

# determine the equations of each side
for $i (0 .. $#@x-1) {
     $m[$i] = ($y[$i+1] - $y[$i])/($x[$i+1] - $x[$i]);
     $b[$i] = $y[$i] - $m[$i]*$x[$i];
}

#test each point
@data_x= (dx1, dx2, dx3, ...);
@data_y= (dy1, dy2, dy3, ...);
for $i (0 .. $#data_y) {
     #find intersection of the line y = $data_y[$i] with each polygon 
+side
     for $j (0 .. $#x-1) {
          if ($m[$j] != 0) {
               $x_intsxn = ($data_y[$i] - $b[$j])/$m[$j];
                    #does that intersection occur within the side's le
+ngth?
                    if (($x_intsxn > $x[$j] && $x_intsxn < $x[$j+1]) |
+| ($x_intsxn > $x[$j] && $x_intsxn < $x[$j+1])) {
                         #is this intersection to the right or left of
+ the point?
                         if ($x_intsxn > $data_x[$i]) {$right++;}
                         else {$left++;}
                     }
           }
     }
     #test odd/even interesections
     if ($left % 2 && $right % 2) {odd number means point inside polyg
+on, do 'insidepoint' stuff or flag point as inside}
     else {flag point as outside}
}
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