在delphi原有控件基础上画图

var
   C:TControlCanvas;
begin
   C := TControlCanvas.Create;
   C.Pen.Color := clRed;
   C.Pen.Width := 3;
   C.Control := Memo1;
   C.MoveTo( 20,20 );
   C.LineTo( 20, 120 );
   C.Free;
end;