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;