[R] How to add a data line (series) to a plot using add=TRUE

[Mark Na]

Hello,

I'd like to use the add=TRUE parameter to add a second data line 
(series) to an existing plot, but R is giving me an error (see below).

This code:
 > rap<-plot(aspen_sort,ylim=c(1,10000),log="y")

...produces the plot to which I'd like to add the second line. But this 
code:
 > rap<-plot(pine_sort,add = TRUE)

...produces this error:

Warning messages:
1: In plot.window(...) : "add" is not a graphical parameter
2: In plot.xy(xy, type, ...) : "add" is not a graphical parameter
3: In axis(side = side, at = at, labels = labels, ...) :
  "add" is not a graphical parameter
4: In axis(side = side, at = at, labels = labels, ...) :
  "add" is not a graphical parameter
5: In box(...) : "add" is not a graphical parameter
6: In title(...) : "add" is not a graphical parameter

I have successfully used add=TRUE (in the same program!) with no errors, 
so I reckon the problem must be related to my data structures, but I 
can't see how.

Any ideas? Thanks!

Mark