\Header{qqmath}{Q-Q Plot with Theoretical Distribution}
\keyword{dplot}{qqmath}
\begin{Description}\relax
Quantile-Quantile plot of a sample and a theoretical distribution
\end{Description}
\begin{Usage}
\begin{verbatim}
qqmath(formula, distribution = qnorm, f.value = NULL, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{formula}] formula of the form \code{\textasciitilde{} x | g1 * g2 * ...},
where \code{x} must be a numeric.

\item[\code{distribution}] a quantile function that takes a vector of
probabilities as argument and produces the corresponding
quantiles. Possible values are \code{qnorm,qunif} etc.
Distributions with other required arguments need to be passed in as
user defined functions. This is also passed in as an argument to the
panel function (though not used by the default panel function), 
and the prepanel function if specified, for use in the bery commonly
used \code{(pre)panel.qqmathline}.

\item[\code{f.value}] optional function of a single integer (representing
sample size), returning a vector of probabilities corresponding to
which quantiles should be plotted. The typical value for this
argument is \code{ppoints}, which is also the S-PLUS default. If
specified, the probabilities generated by this function is used for
the plotted quantiles, using the \code{quantile} function for the
sample, and the \code{distribution} argument for the theoretical
distribution.

\code{f.value} defaults to \code{NULL}, which has the effect of
using \code{ppoints} for the quantiles of the theoretical
distribution, but the exact data values for the sample. This is
similar to what happens for \code{qqnorm}.


\item[\code{...}] Other arguments
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{qqmath} produces a Q-Q plot of the given sample and a
theoretical distribution.  The default behaviour of \code{qqmath} is
different from the corresponding S-PLUS function, but is similar to
\code{qqnorm}. See the entry for \code{f.value} for specifics.

This and all other high level Trellis functions have several
arguments in common. These are extensively documented only in the
help page for \code{xyplot}, which should be consulted to learn more
detailed usage.
\end{Details}
\begin{Value}
An object of class ``trellis''. The `update' method can be used to
update components of the object and the `print' method (usually called
by default) will plot it on an appropriate plotting device.
\end{Value}
\begin{Author}\relax
Deepayan Sarkar \url{deepayan@stat.wisc.edu}
\end{Author}
\begin{SeeAlso}\relax
\code{\Link{xyplot}}, \code{\Link{panel.qqmath}},
\code{\Link{panel.qqmathline}}, \code{\Link{prepanel.qqmathline}},
\code{\Link{Lattice}}, \code{\Link{quantile}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
qqmath(~ rnorm(100), distribution = function(p) qt(p, df = 10))
data(singer)
qqmath(~ height | voice.part, aspect = 1, data = singer,
       prepanel = prepanel.qqmathline,
       panel = function(x, y) {
          panel.qqmathline(y, distribution = qnorm)
          panel.qqmath(x, y)
       })
\end{ExampleCode}
\end{Examples}

