Suppose I create a multiple plot with zoo, using:

index <- ISOdatetime(2004, rep(1:2, 5), sample(28, 10), 0, 0, 0)
foo <- zoo(rnorm(10), index)
for (i in 1:9) {
  data <- rnorm(10)
  z1 <- zoo(data, index)
  foo <- cbind(foo, z1)
}
plot(foo)

This creates 10 plots on one device, one for each column in foo.

Now I want to go back and use abline to draw a line at the mean on each of
my 10 plots. How do I select the appropriate set of axes to draw my line on?

Thanks,
Jonathan

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