Выдает Nan - Turbo Pascal

 program UpdateGraph;
 uses Graph,Crt;
 var xMax,yMax,i:integer;
     st:string[10];
       x:extended;
 
 procedure GraphInit;
 var
  grDriver,grMode,ErrCode: integer;
 begin
  grDriver := Detect;
  InitGraph(grDriver, grMode,'');
  ErrCode := GraphResult;
  if ErrCode <> grOk then
   begin
    Writeln('Graphics error:', GraphErrorMsg(ErrCode));
    ReadKey;
    Halt;
   end;
  xMax:=GetMaxX; yMax:=GetMaxY;
 end;
 
 procedure Body;{MainForm}
 begin
  Line(xMax-140,3,xMax-140,yMax-3);
  Line(62,450,xMax-140,450);
  OutTextXY(508,10,'y = a*cos(x)+b*x');
  OutTextXY(507,300,'a =');
  OutTextXY(507,317,'b =');
 
  OutTextXY(386,461,'Close');
  Rectangle(380,455,430,474);
  OutTextXY(186,461,'Input');
  Rectangle(180,455,230,474);
  OutTextXY(266,461,'Clear');
  Rectangle(260,455,310,474);
 
  OutTextXY(536,34,'x');
  OutTextXY(590,34,'y');
  Line(516,45,623,45);
  Line(562,30,562,270);
  Rectangle(xMax-123,30,xMax-16,270);
 end;
 
 procedure Coord;{CoordinateNet}
 var
  NumHoriz,NumVert:string[10];
 begin
  for i:=-10 to 10 do
   begin
    Str(i,NumHoriz);
    case i of
     0..9:OutTextXY(270+(21*i),438,NumHoriz);
     10:OutTextXY(475,438,NumHoriz);
     -9..-1:OutTextXY(262+(21*i),438,NumHoriz);
    end;
   end;
 
   for i:=-10 to 10 do
    begin
     Str(i*10,NumVert);
     case i of
     -9..-1:OutTextXY(33,221+(-21*i),NumVert);
     0:OutTextXY(48,222,NumVert);
     1..9:OutTextXY(40,220+(-21*i),NumVert);
     10:OutTextXY(33,12,NumVert);
     end;
    end;
 
 SetColor(DarkGray);
 Rectangle(62,14,482,433);
 
              {Vertical lines}
   for i:=1 to 20 do
   begin
    case i of
     1..10:Line(41+(21*i),14,41+(21*i),433);
     12..19:Line(41+(21*i),14,41+(21*i),433);
     20:Line(41+(21*i),35,41+(21*i),433);
    end;
   end;
              {Horizontal lines}
   for i:=1 to 19 do
   begin
    case i of
     1..9:Line(63,14+(21*i),482,14+(21*i));
     11..19:Line(63,14+(21*i),482,14+(21*i));
    end;
   end;
 
 SetColor(LightGreen);
 Line(272,14,272,433);
 Line(62,225,482,225);
 SetColor(White);
 end;
 
 procedure xValues;
 begin
  for i:=1 to 21  do
   begin
   x:=-11+i;
   Str(x:2:0,st);
   OutTextXY(530,44+(10*i),st);
   end;
 
  end;
 
 Begin
 
  GraphInit;
  Body;
  Coord;
  xValues;
  ReadKey;
 
 End.