I use to recommend adding a dummy series with ShowInLegend set to False.
In this case this should do it:
Chart1.Draw;
Chart1.Axes.Left.Automatic:=False;
Chart1.Axes.Right.Automatic:=False;
with Chart1.AddSeries(TLineSeries) do
begin
ShowInLegend:=False;
VertAxis:=aBothVertAxis;
end;