[R-SIG-Finance] Missing tick mark labels from barChart / quantmod - plot.xts missing

[风]

Dear Jeff and R experts,
I wonder whether you have received the previous email on this matter.
I have narrow the problem to plot.xts missing.

>require(quantmod)            
Loading required package: quantmod
Loading required package: xts
Loading required package: zoo
Attaching package: 'zoo'
        The following object(s) are masked from package:base :
         as.Date.numeric 
Loading required package: Defaults
quantmod: Quantitative Financial Modelling Framework
Version 0.3-6, Revision 433
http://www.quantmod.com
       
>getSymbols('AAPL')   
[1] "AAPL"
>barChart(AAPL)
#successful.  But NO date label for x axis labels.
>a<-Cl(AAPL)
>class(a)
[1] "xts" "zoo"
>plot.zoo(a)
#successful.  There are annual label for x axis major ticks.
>chartSeries(a)
Error in sum(poss.new) : invalid 'type' (list) of argument
>plot(a)
#no date label for x  axis ticks
>plot(as.xts(a))
#same graph as the above. still no date label for x axis ticks
>plot.xts(a)
Error: could not find function "plot.xts"
> ?plot.xts
#the help window for plot.xts poped up.

>library(help=xts)
#window poped up with the following content:
Version:       0.0-15
Revision:      139
Date:          2007-06-04
Author:        Jeffrey A. Ryan, Josh M. Ulrich
Depends:       zoo
...
License:       GPL-3
URL:           http://r-forge.r-project.org/projects/xts/
Packaged:      Mon Jun 9 23:47:46 2008; jryan
Built:         R 2.7.2; i386-pc-mingw32; 2008-10-15 23:19:56; windows
Index:
...
plot.xts                Plotting xts Objects
...

It seems that plot.zoo does its job but plot.xts may have some problem.

Besides, the OS is Vista Oem on a notebook.  R is 2.7.2 or 2.8.0.

I wonder whether it is enough for R experts to diagnose the problem.
Thanks.

Wind