{Philippe Martinole, http://wiki.lazarus.freepascal.org/TAChart}

procedure TForm1.Button12Click(Sender: TObject);
var
  TALine:TTALine;
begin
TALine:=TTALine.Create(TAChart1);
TAChart1.AddSerie(TALine);
TALine.LineStyle:=lsHorizontal;
TALine.Position:=10.231;
TALine.Visible:=True;
end;k