[R] synchronisation of time series data using interpolation

[e-letter]

I created separate text files for the 2 data sets. I enter the
following comands:

library(zoo)
library(chron)
z1<-read.zoo(textConnection("/path/to/test1.txt"),header=FALSE,sep=",",FUN=times)
z2<-read.zoo(textConnection("/path/to/test2.txt"),header=FALSE,sep=",",FUN=times)
z3<-window(na.approx(merge(z1,z2)),time(z1))
plot(z3$z1,z3$z2)
Error in plot.window(xlim, ylim, log, asp, ...) :
        need finite 'xlim' values
In addition: Warning messages:
1: no non-missing arguments to min; returning Inf in: min(x)
2: no non-missing arguments to max; returning -Inf in: max(x)
3: no non-missing arguments to min; returning Inf in: min(x)
4: no non-missing arguments to max; returning -Inf in: max(x)

The resultant graph window was blank, so I entered the following command

plot(z3$z1,z3$z2,xlim=c(0,100),ylim=c(0,100))

The graph window showed y axis (labelled 'z3$z1') and x axis (labelled 'Index').

I do not understand the instruction "...to use window to pick off..."