[R] Plotting monthly timeseries with an x-axis in "time format"

[vittorio]

I have the following timeseries "tab"
=====================================
> str(tab)
 mts [1:23, 1:2] 79.5 89.1 84.9 75.7 72.8 ...
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : chr [1:2] "Ipex...I" "Omel...E"
 - attr(*, "tsp")= num [1:3] 2006 2008   12
 - attr(*, "class")= chr [1:2] "mts" "ts"

> tab
         Ipex...I Omel...E
Jan 2006    79.47    77.95
Feb 2006    89.13    76.73
Mar 2006    84.86    51.20
Apr 2006    75.68    51.86
May 2006    72.82    51.29
Jun 2006    78.87    49.45
Jul 2006    93.46    53.88
Aug 2006    78.18    47.96
Sep 2006    82.46    55.07
Oct 2006    77.25    45.34
Nov 2006    80.95    37.07
Dec 2006    84.39    37.53
Jan 2007    81.70    47.79
Feb 2007    74.76    37.50
Mar 2007    65.29    30.35
Apr 2007    60.30    37.78
May 2007    66.59    34.13
Jun 2007    73.19    39.14
Jul 2007    92.39    39.89
Aug 2007    65.76    35.46
Sep 2007    77.45    36.54
Oct 2007    74.22    38.39
Nov 2007   101.36    47.33
Dec 2007   100.01  45.34
===============================

Plotting tab with a simple "plot(tab,plot.type="single")" I'm obtaining a 
graph  with the x axis in an orrible decimal format so that,e.g., Jan 2006 is 
2006.0 and Nov 2006 is 2006.8(33)!

 Instead I would like to see the x-axis in a more human-readable format, for 
instance, 12 tics for each year and a label at the beginning of each quarter 
of the year: 2006.1, 2006.4,2006.7.
 - OR -
more elegantly, I would like to have the 12 tics with the month shortened 
labels: Jan, Feb, etc. and below, say June, one label for the year.

Please help.

Ciao
Vittorio