[R] newbie needs help plotting time series

[Kathi]

Dear R help list

I have recently started using R to visualize time series of water level measurements. The code I wrote 
does exactly what I want:

plot(x, a$B112, axes=FALSE, ylim=c(405,410), col="red", pch=15, 
type="o", ylab="metres above sea level", main="Main Plot Title")
axis(2, at=405:410, tck=1, col="gray60")
axis.Date(1, at=seq(as.Date("2004-01-01"), as.Date("2008-06-01"), 
by="month"), labels=seq(as.Date("2004-01-01"), as.Date("2008-06-01"), 
by="month"), tck=1, col="gray60")

Now I wanted to shift the data visualized by 6 months, so I replaced "2004-01-01" by "2004-07-01" 
and "2008-06-01" by "2008-12-01". Instead of the desired plot, I get an error message stating 
(forgive the perhaps faulty translation into English, I'm working on a German version of R):

mistake in axis(side, at = z, labels = labels, ...) : 
  length of 'at' and 'labels' different, 50 != 54

What have I done wrong? Thank you very much for your help!

Katharina Dubach