\HeaderA{qq}{Quantile-Quantile Plots of Two Samples}{qq}
\methaliasA{qq.formula}{qq}{qq.formula}
\keyword{dplot}{qq}
\begin{Description}\relax
Quantile-Quantile plots for comparing two Distributions
\end{Description}
\begin{Usage}
\begin{verbatim}
qq(x, data, ...)

## S3 method for class 'formula':
qq(x, data, aspect = "fill", 
    panel = panel.qq, prepanel, scales, strip, 
    groups, xlab, xlim, ylab, ylim, f.value = NULL, 
    drop.unused.levels = lattice.getOption("drop.unused.levels"),
    ...,
    qtype = 7,
    default.scales = list(),
    subscripts,
    subset)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] The object on which method dispatch is carried out.

For the formula method, a formula of the form \code{y \textasciitilde{} x | g1 * g2
      * ...}, where \code{x} must be a numeric, and \code{y} can be a
factor, shingle, character or numeric vector, with the restriction
that there must be exactly two levels of \code{y}, which divide the
values of \code{x} into two groups. Quantiles for these groups will
be plotted along the two axes.

\item[\code{data}] For the \code{formula} methods, an optional data frame in which
variables in the formula (as well as \code{groups} and
\code{subset}, if any) are to be evaluated.

\item[\code{f.value}] optional numeric vector of probabilities, quantiles
corresponding to which should be plotted.  Can also be a function of
a single integer (representing sample size) that returns such a
numeric vector.  The typical value for this argument is the function
\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.

\code{f.value} defaults to \code{NULL}, which is equivalent to
using \code{function(n) ppoints(n, a = 1)}. This has the effect of
including the minimum and maximum data values in the computed
quantiles. This is similar to what happens for \code{qqplot} but
different from the default \code{qq} behaviour in S-PLUS.

For large data, this argument can be useful in plotting a smaller
set of quantiles, which is usually enough to capture the pattern.

\item[\code{panel}] The function that uses the packet (subset of display
variables) corresponding to a panel to create a display.  Default
panel functions are documented separately, and often have arguments
that can be used to customize its display in various ways.  Such
arguments can usually be directly supplied to the high level
function.  
\item[\code{qtype}] \code{type} argument for the \code{\LinkA{quantile}{quantile}} 
\item[\code{aspect, prepanel, scales,
strip, groups, xlab, xlim, ylab, ylim, drop.unused.levels,
default.scales, subscripts, subset}] See \code{\LinkA{xyplot}{xyplot}} 
\item[\code{...}] Further arguments.  See corresponding entry in \code{\LinkA{xyplot}{xyplot}}
for non-trivial details.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{qq} produces a Q-Q plot of two samples.  The default behaviour
of \code{qq} is different from the corresponding S-PLUS function. 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 \code{"trellis"}. The
\code{\LinkA{update}{update}} method can be used to
update components of the object and the
\code{\LinkA{print}{print}} method (usually called by
default) will plot it on an appropriate plotting device.
\end{Value}
\begin{Author}\relax
Deepayan Sarkar \email{Deepayan.Sarkar@R-project.org}
\end{Author}
\begin{SeeAlso}\relax
\code{\LinkA{xyplot}{xyplot}}, \code{\LinkA{panel.qq}{panel.qq}},
\code{\LinkA{qqmath}{qqmath}}, \code{\LinkA{Lattice}{Lattice}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
qq(voice.part ~ height, aspect = 1, data = singer,
   subset = (voice.part == "Bass 2" | voice.part == "Tenor 1"))
\end{ExampleCode}
\end{Examples}

