[R] Plotting multiple time series with variables having different units

[David Parkhurst]

I've tried to figure out how to do this from what I read, but haven't 
been successful.  Suppose I have a dataframe with variables Date, X, and 
Y (and maybe U, V, and Z) where X, Y, etc. have different units.  I'd 
like to plot Y vs. Time above X vs. Time, above one another.

For example, X is the number of gulls counted on a reservoir, and Y is 
the number of coliform bacteria counted on a petri plate from a water 
sample leaving the reservoir, so these have very different ranges. U and 
V might be numbers of geese and numbers of ducks counted on the same days.

What commands would I use to create such a set of plots?
Thank you for any help.
David