[Delphi] Differential of Function (just for self documentation)

Here the code, it have the graph of function and plot of it diferential (gradien) line

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls;

type
  TForm1 = class(TForm)
    Image1: TImage;
    Timer1: TTimer;
    procedure proses;
    procedure awal;
    function f(x:real):real;
    function df(x:real):real;
    function fs(x,xs,ys,m:real):real;
    procedure gambar;
    procedure Timer1Timer(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  dx:real=0.01;
  x0,y0:integer;
implementation

{$R *.dfm}

procedure tform1.awal;
begin
  {gambar sumbu}
  x0:=round(image1.Width/2);
  y0:=round(image1.Height/2);
  with image1.Canvas do begin
    moveto(0,y0);
    lineto(image1.Width,y0);
    moveto(x0,0);
    lineto(x0,image1.Height);
  end;
end;
function tform1.f(x:real):real;
begin
  f:=10*cos(x)+x;
end;
function tform1.df(x:real):real;
begin
  df:=(f(x+dx)-f(x))/dx;
end;
function tform1.fs(x,xs,ys,m:real):real;
var c:real;
begin
  c:=ys-m*xs;
  fs:=m*x+c;
end;

procedure tform1.gambar;
var i:integer;
x,y,x1,y1,m:real;
begin
//  image1.Canvas.TextOut(0,0,'f(x)=x^2-2x-8 memiliki gradien '+floattostrf(df(0),ffnumber,8,2)+' di x=0');
  image1.Canvas.TextOut(0,0,'f(x)=x^2-2x-8 memiliki gradien '+format('%8.2f',[df(0)])+' di x=0');
  with image1.Canvas do begin
    {gambar fungsi}
    for i:=-200 to 200 do begin
      x:=i/8;
      y:=f(x);
      x1:=(i+1)/8;
      y1:=f(x1);
      pen.Color:=clblue;
      moveto(x0+i,y0-round(y));
      lineto(x0+i+1,y0-round(y1));
    end;
    {gambar garis singgung}
    m:=df(0);
    for i:=-200 to 200 do begin
      x:=i/8;
      y:=fs(x,0,f(0),m);
      x1:=(i+1)/8;
      y1:=fs(x1,0,f(0),m);
      pen.Color:=cllime;
      moveto(x0+i,y0-round(y));
      lineto(x0+i+1,y0-round(y1));
    end;
  end;
end;
procedure tform1.proses;
begin
  awal;
  gambar;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  timer1.Enabled:=false;
  proses;
end;

end.