As the customer explains in the forums, scrolling with the mouse wheel doesn't trigger the OnScroll event.

Note it is triggered when using the TAxisArrowTool and the TAxisScrollTool.

Example using a memo to see how many times the OnScroll event is triggered:

uses Chart, Series, TeeProcs, TeeTools;

var Chart1: TChart;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Chart1:=TChart.Create(Self);

  with Chart1 do
  begin
    Parent:=Self;
    Align:=alClient;
    Color:=clWhite;
    Gradient.Visible:=False;
    Walls.Back.Color:=clWhite;
    Walls.Back.Gradient.Visible:=False;
    Legend.Hide;
    View3D:=False;

    AddSeries(TLineSeries).FillSampleValues;

    OnScroll:=ChartOnScroll;

    with TAxisArrowTool(Tools.Add(TAxisArrowTool)) do
    begin
      Axis:=Axes.Left;
    end;

    with TAxisScrollTool(Tools.Add(TAxisScrollTool)) do
    begin
      Axis:=Axes.Bottom;
    end;
  end;

  Memo1.Clear;
end;

procedure TForm1.ChartOnScroll(Sender: TObject);
begin
  Memo1.Lines.Add('onscroll');
end;