[R] plot histogram and print

[livia]

I have got a dataset with two factors, and I would like to print the
histograms of the variable "return" for each combination of the two factors.
I do not know how can I name the figure title to sth like "main="alevel
&blevel"" according to the data.

par(ask=TRUE)

myhistogram <- function(x)
{
hist (x, freq = FALSE)
lines (density(x), col = "red") 
rug (x)
}
tapply(return, list(a, b), myhistogram)

Could anyone help me with this? Many thanks.
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