To get the label text of the just clicked value (in your case a vertical bar), you can inspire in the following method written for the OnClickSeries
event:
procedure TForm1.Chart1ClickSeries(Sender: TCustomChart; Series: TChartSeries;
ValueIndex: Integer; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
ShowMessage(Series.Labels[ValueIndex]);
end;