GraphАВС: ошибка в программе, график не получается

program A1;
Uses GraphABC;
var gX,gy,d,m,i,j:integer;
F:text;
Xmin, Xmax, Ymin, Ymax, Y, X:real;
S:string;
begin
assign(f,'d:\trigon1.txt');
Rewrite(F);
Xmin:=0;
Xmax:=2*pi;
Ymax:=-2;
Ymin:=2;
writeln(f,'X ','Y ');
X:=0;
while X<=2*pi do
begin
Y:=sin(2*x)+cos(2*x);
if (x>=pi) and (x<=1.5*pi) then
if Y<0 then Y:=-1;
writeln (f,X:4:2,' ',Y:4:2);
if Y>Ymax then Ymax:=Y;
if Y<Ymin then Ymin:=Y;
X:=X+(2*pi/30);
end;
writeln(f,'Ymax',Ymax:4:2);
writeln(f,'Ymin',Ymin:4:2);
close(F);
line(19,0,19,479);
line(19,239,639,239);
line(19,0,14,10);
line(19,0,24,10);
line(639,239,629,234);
line(639,239,629,244);
J:=2;
Gx:=19;
Gy:=79;
while gy<=479-80 do
begin
line(gx-5,gy,gx+5,gy);
str(j,s);
textout(gx-19,gy,s);
j:=j-1;
Gy:=gy+80;
end;
Gx:=19;
gy:=239;
while gx<639 do
begin
line(gx,gy-5,gx,gy+5);
gx:=gx+20;
end;
s:=' ';
s:=chr(210);
textout(314,259,s);
s:='2'+s;
textout(gx-20,259,s);
gy:=479+round((-479/6)*4);
moveto(19,gy);
for i:=19 to 619 do
begin
x:=(i*19)*(2*pi+(2*pi/30))/620;
y:=sin(2*x)+cos(2*x);
if (x>pi) and (x<1.5*pi) then
if y<0 then y:=-1;
gy:=479+round((-479/6)*(y+3));
setpixel(i,gy,clblack);
end;
readln;
end.