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var i:Integer;

  ...
  with Chart1 do
  begin
    Series[0].Clear;
    Series[1].Clear;

    for i:=1 to 12 do
    begin
      Series[0].AddXY(i, Random(100));
      Series[1].AddXY(i, Random(100));
    end; // for i
  end; // for i
  ...					

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unit test;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls, TeeProcs, TeEngine, Chart;

type
  TForm1 = class(TForm)
    Chart1: TChart;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

USES StatMod;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var i : integer;
begin
  chart1.Series[0].Clear;
  for i:= 1 to 12 do 
    chart1.Series[0].AddXY(i, Random(100));
end;

end.					

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type
  TForm1 = class(TForm)
    Button1: TButton;
    Chart1: TChart;
    Series1: TLineSeries;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;					

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//     Series[1].Clear;
      for i:=1 to 12 do 
        Series[1].AddXY(i, Random(100));