histogram method for S4 class.

Hi,

I'm trying to develop an histogram method for a class called "FLQuant"
which is used by the package FLCore (http://flr-project.org). FLQuant is
an extension to "array". There is an as.data.frame method that coerces
flquant into a data.frame suitable for lattice plotting. The problem is
that when I coerce the object and plot it after it works but if the
method is applied within the histogram method it does not work. See the
code below (the FLCore package is here
http://prdownloads.sourceforge.net/flr/FLCore_1.0-1.tar.gz?download)

library(FLCore)

Loading required package: lattice

data(ple4)
histogram(~data|year, data=ple4 at catch.n)

Error in inherits(x, "factor") : Object "x" not found

histogram(~data|year, data=as.data.frame(ple4 at catch.n))

The catch.n slot is a FLQuant object and the code for histogram is the
following

setMethod("histogram", signature(formula="formula", data="FLQuant"),
    function (formula, data = parent.frame(), allow.multiple =
is.null(groups) || outer,
        outer = FALSE, auto.key = FALSE, aspect = "fill", panel =
"panel.histogram", prepanel = NULL,
        scales = list(), strip = TRUE, groups = NULL, xlab, xlim, ylab,
ylim,
        type = c("percent", "count", "density"),
        nint = if (is.factor(x)) length(levels(x)) else
round(log2(length(x)) + 1),
        endpoints = extend.limits(range(x[!is.na(x)]), prop = 0.04),
        breaks = if (is.factor(x)) seq(0.5, length = length(levels(x)) +
1) else do.breaks(endpoints, nint),
        equal.widths = TRUE, drop.unused.levels =
lattice.getOption("drop.unused.levels"), ...,
        default.scales = list(), subscripts = !is.null(groups), subset =
TRUE) {

        qdf <- as.data.frame(data)

        histogram(formula, data = qdf, allow.multiple = allow.multiple,
outer = outer,
            auto.key = auto.key, aspect = aspect, panel = panel,
prepanel = prepanel, scales = scales,
            strip = strip, groups = groups, xlab=xlab, xlim=xlim,
ylab=ylab, ylim=ylim, type = type,
            nint = nint, endpoints = endpoints, breaks = breaks,
equal.widths = equal.widths,
            drop.unused.levels = drop.unused.levels, ..., default.scales
= default.scales,
            subscripts = subscripts, subset = subset)
    }
)


Any ideas ?