怎样在DbGrid的左边，实现像EXCEL那样的自动编号？

unit Unit1;

interface

uses
 Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
 Grids, DBGrids, StdCtrls, Buttons, Db, DBTables, ExtCtrls, jpeg;
const ROWCNT=20;

type
    tmygrid=class(tdbgrid)
    protected
      procedure Paint;override;
      procedure DrawCell(ACol:Integer;ARow:Integer;ARect:TRect;AState:TGridDrawState);override;
    public
      constructor create(AOwner:TComponent);override;
      destructor  destroy;override;
    end;

 TForm1 = class(TForm)
   BitBtn1: TBitBtn;
   DataSource1: TDataSource;
   Table1: TTable;
   procedure BitBtn1Click(Sender: TObject);
 private
   { Private declarations }
 public
   { Public declarations }
 end;

var
 Form1: TForm1;
 mygrid:tmygrid;
implementation

{$R *.DFM}

    {tmygrid}
    constructor tmygrid.create(AOwner:TComponent);
    begin
       inherited create(Owner);
       RowCount:=ROWCNT;
    end;

    destructor tmygrid.destroy;
    begin
      inherited;
    end;

    procedure tmygrid.Paint;
    begin
      RowCount:=ROWCNT;
      if dgIndicator in options then
         ColWidths[0]:=30;
      inherited;
    end;

    procedure tmygrid.DrawCell(ACol:Integer;ARow:Integer;ARect:TRect;AState:TGridDrawState);
    begin
      inherited;
      if (ARow>=1) and (ACol=0) then
         Canvas.TextRect(ARect,ARect.Left,ARect.Top,IntToSTr(ARow));
   end;

procedure TForm1.BitBtn1Click(Sender: TObject);
begin
 mygrid:=tmygrid.create(Self);
 mygrid.parent:=self;
 mygrid.left:=0;
 mygrid.top:=0;
 mygrid.Height:=300;
 mygrid.DataSource:=DataSource1;
end;

end.