procedure TForm1.Button3Click(Sender: TObject);
var x, y: array[1..50] of double;
 i: integer;
 scalex, scaley, ymin, ymax, xmin, xmax: double;
begin
 for i := 1 to 50 do
  begin
  y[i] := sin(i * 0.5);
  x[i] := i;
  end;
 xmin := x[1];
 xmax := x[1];
 ymin := y[1];
 ymax := y[1];
 for i := 2 to 50 do
  begin // или используйте ymin:=MinValue(y); и т.д.
  if y[i] < ymin then ymin := y[i];
  if y[i] > ymax then ymax := y[i];
  if x[i] < xmin then xmin := x[i];
  if x[i] > xmax then xmax := x[i];
  end;
 scalex := paintbox1.Width / (xmax - xmin);
 scaley := paintbox1.Height / (ymax - ymin);
 with paintbox1.canvas do
  begin
  moveto(trunc(scalex * (x[1] - xmin)), paintbox1.height - trunc(scaley * (y[1] - ymin)));
  for i := 2 to 50 do
  Lineto(trunc(scalex * (x[i] - xmin)), paintbox1.height - trunc(scaley * (y[i] - ymin)));
  end;
end;

for i:=1 to 500 do begin

 paintbox1.Canvas.Pixels [round(sin(i*5)*10+50),round(sin(i*10)*10+50)] := RGB(0,0,0);