Well i've made a post about this (I think...) But seems like it's gone ;_; So i have to repost... I've got this so far using Forum_account MousePosition library.
icon = 'Stylus.dmi';
icon_state = "Stylus";
screen_loc = "1,1"
icon = 'trf.dmi';
MouseUpdate(tx, px, ty, py)
var/mob/A = src.mob;
A.MyStylus.screen_loc = "[tx]:[px - 16],[ty]:[py - 16]"
var/obj/Capturing/Stylus/Stylus = new();
mob.MyStylus = Stylus;
src.screen += Stylus;
What i'm missing is the line drawing (Which in my oppinion is a little hard depending on the direction.)
Something like this: http://youtu.be/eFbqywlRZa8?t=3m22s
(How line is begin drawed... and how the system checks that pokemon is inside the "circle", i've been looking and seems like this is done by some maths.)