procedure TForm1.Button1Click(Sender: TObject);
var
  MousePos: TPoint;
begin
  MousePos := Chart1.ScreenToClient(Mouse.CursorPos);

  // in MousePos.X is horizontal position of the cursor relative to the chart, just like X in OnMouseMove
  // in MousePos.Y is vertical position of the cursor relative to the chart, just like Y in OnMouseMove
end;