[R-SIG-Finance] Missing tick mark labels from barChart / quantmod

[风]

I guess it maybe a silly question, but I have spent several hours reading help and googling and still could not find the missing  labels for x axis ticks.   Or tick marks?   Anyway, I mean, the year/month names along the x axis. :)
With the following codes, I got charts exactly I want. Except that there is no label for x axis ticks.  There are ticks line indeed .
I guess maybe there are something wrong with the par() of my R.  But I just cannot figure out from reading the help(par).    Indeed, if I use the plot to draw some chart for simple data, which plot maybe not the plot.xts I guess,  there are tick labels for x axis. 
Would you please tell me where could I find those tick labels for barChart?

require(quantmod)
getSymbols("AAPL")
barChart(AAPL)