Рисование графика: y=200-x^2

uses graphABC;
function f(x:real):real;
begin
f:=200-x*x;
end;
var xn,xk,x,dx,m:real;
    x0,y0,i:integer;
begin
//интервал
xn:=-20;
xk:=20;
setwindowsize(500,500);
centerwindow;
//начало координат
x0:=windowwidth div 2;
y0:=windowheight div 2;
//масштаб
m:=(x0-20)/xk;
//оси
line(0,y0,windowwidth,y0);
textout(windowwidth-20,y0-20,'X');
line(x0,0,x0,2*y0);
textout(x0+5,0,'Y');
textout(x0+5,y0+10,'0');
//засечки и подписи по осям
for i:=1 to trunc(xk) do
if i mod 2=0 then
 begin
  line(x0+round(i*m),y0-3,x0+round(i*m),y0+3);
  line(x0-round(i*m),y0-3,x0-round(i*m),y0+3);
  line(x0-3,y0-round(i*m),x0+3,y0-round(i*m));
  line(x0-3,y0+round(i*m),x0+3,y0+round(i*m));
  textout(x0+round(i*m),y0+10,inttostr(i));
  textout(x0-round(i*m),y0+10,inttostr(-i));
  textout(x0-25,y0-round(i*m)-5,inttostr(i*10));
  textout(x0-30,y0+round(i*m)-5,inttostr(-i*10));
 end;
//график
setpencolor(clBlue);
x:=xn;
dx:=0.01;
while x<=xk do
 begin
  if x=xn then moveto(x0+round(x*m),y0-round(f(x)/10*m))
  else lineto(x0+round(x*m),y0-round(f(x)/10*m));
  x:=x+dx;
 end;
setfontcolor(clBlue);
setfontsize(12);
setfontstyle(fsBold);
setbrushstyle(bsClear);
textout(100,10,'y=200-x^2');
end.