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 procedure StringGrid1DrawCell(Sender: TObject; ACol, ARow: Integer;
      Rect: TRect; State: TGridDrawState);
...
Procedure TForm1.StringGrid1DrawCell(Sender: TObject; aCol, aRow: Integer;
  Rect: TRect; State: TGridDrawState);
Begin
  With Sender As TStringGrid Do
  Begin
   If (aCol < FixedCols) Or (aRow < FixedRows) Then Exit;  // Wenn du auf "gesperrten" Feldern nicht zeichnen willst
   Canvas.moveto(90,100);
   Canvas.LineTo(100,100);
  end;
end;

Procedure Tform1.Button1Click(Sender: Tobject);
begin
Stringgrid1.repaint;
end;