[R] Strange behavior with time-series x-axis

[Allen McIntosh]

I recently called plot(x,y) where x was an array of POSIXct timestamps, 
and was pleasantly surprised that it produced a nice plot right out of 
the box:

z <- as.POSIXct(c("2006-10-26 08:00:00 EDT","2007-10-25 12:00:00 EDT"))
x <- seq(z[1],z[2],len=100)
y <- 1:100
plot(x,y,type="l")

The X axis had nice labels, one tick mark every other month.  (Plotting 
on a 1024x768 X11 display, Fedora Core 5, R version 2.5.1.)

What I really wanted to do was a little more elaborate, involving 
multiple Y variables, so I did the following to draw the axes and frame 
the plot:

plot(range(x),range(y),type="n")

and suddenly the nice X axis labels were not so nice anymore.  There 
were three tickmarks labeled "Nov", "Oct" and "Nov", and that's all.

(range(x) strips the tzone attribute from x, but replacing range(x) with 
c(x[1],x[100]) doesn't change anything).

If x covers a slightly larger span of time, e.g.

z <- as.POSIXct(c("2006-10-1 08:00:00 EDT","2007-10-25 12:00:00 EDT"))
x <- seq(z[1],z[2],len=100)
plot(x,y,type="l")

then the x axis labeling is also problematic, consisting of a single 
tick at "2007".

Is there anything I can do about this, short of turning off automatic X 
labels and doing things myself?