\HeaderA{plot.boot}{Plots of the Output of a Bootstrap Simulation}{plot.boot}
\keyword{hplot}{plot.boot}
\keyword{nonparametric}{plot.boot}
\begin{Description}\relax
This takes a bootstrap object and produces plots for the bootstrap
replicates of the variable of interest.
\end{Description}
\begin{Usage}
\begin{verbatim}
## S3 method for class 'boot':
plot(x, index=1, t0=NULL, t=NULL, jack=FALSE, qdist="norm", 
     nclass=NULL, df, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] An object of class \code{"boot"} returned from one of the bootstrap generation
functions.

\item[\code{index}] The index of the variable of interest within the output of \code{boot.out}.  This
is ignored if \code{t} and \code{t0} are supplied.

\item[\code{t0}] The original value of the statistic.  This defaults to \code{boot.out\$t0[index]}
unless \code{t} is supplied when it defaults to \code{NULL}. In that case no vertical 
line is drawn on the histogram.

\item[\code{t}] The bootstrap replicates of the statistic.  Usually this will take on its
default value of \code{boot.out\$t[,index]}, however it may be useful sometimes 
to supply a different set of values which are a function of \code{boot.out\$t}.

\item[\code{jack}] A logical value indicating whether a jackknife-after-bootstrap plot is 
required.  The default is not to produce such a plot.

\item[\code{qdist}] The distribution against which the Q-Q plot should be drawn.  At present 
\code{"norm"} (normal distribution - the default) and \code{"chisq"} (chi-squared 
distribution) are the only possible values.

\item[\code{nclass}] An integer giving the number of classes to be used in the bootstrap histogram.
The default is the integer between 10 and 100 closest to
\code{ceiling(length(t)/25)}. 

\item[\code{df}] If \code{qdist} is \code{"chisq"} then this is the degrees of freedom for the chi-squared
distribution to be used.  It is a required argument in that case.

\item[\code{...}] When \code{jack} is \code{TRUE} additional parameters to \code{jack.after.boot} can be 
supplied.  See the help file for \code{jack.after.boot} for details of the
possible parameters.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
This function will generally produce two side-by-side plots.  The left plot
will be a histogram of the bootstrap replicates.  Usually the breaks of the
histogram will be chosen so that \code{t0} is at a breakpoint and all intervals
are of equal length.  A vertical dotted line indicates the position of \code{t0}.
This cannot be done if \code{t} is supplied but \code{t0} is not and so, in that case, 
the breakpoints are computed by \code{hist} using the \code{nclass} argument and no
vertical line is drawn.


The second plot is a Q-Q plot of the bootstrap replicates.  The order 
statistics
of the replicates can be plotted against normal or chi-squared quantiles.  In
either case the expected line is also plotted.  For the normal, this will
have intercept \code{mean(t)} and slope \code{sqrt(var(t))} while for the chi-squared
it has intercept 0 and slope 1.  


If \code{jack} is \code{TRUE} a third plot is produced beneath these two.  That plot
is the jackknife-after-bootstrap plot.  This plot may only be requested
when nonparametric simulation has been used.  See \code{jack.after.boot} for further
details of this plot.
\end{Details}
\begin{Value}
\code{boot.out} is returned invisibly.
\end{Value}
\begin{Section}{Side Effects}
All screens are closed and cleared and a number of plots are produced on the 
current graphics device.  Screens are closed but not cleared at termination
of this function.
\end{Section}
\begin{SeeAlso}\relax
\code{\LinkA{boot}{boot}}, \code{\LinkA{jack.after.boot}{jack.after.boot}}, \code{\LinkA{print.boot}{print.boot}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
# We fit an exponential model to the air-conditioning data and use
# that for a parametric bootstrap.  Then we look at plots of the
# resampled means.
air.rg <- function(data, mle)
     rexp(length(data), 1/mle)

air.boot <- boot(aircondit$hours, mean, R=999, sim="parametric",
                 ran.gen=air.rg, mle=mean(aircondit$hours))
plot(air.boot)

# In the difference of means example for the last two series of the 
# gravity data
grav1 <- gravity[as.numeric(gravity[,2])>=7,]
grav.fun <- function(dat, w)
{    strata <- tapply(dat[, 2], as.numeric(dat[, 2]))
     d <- dat[, 1]
     ns <- tabulate(strata)
     w <- w/tapply(w, strata, sum)[strata]
     mns <- tapply(d * w, strata, sum)
     mn2 <- tapply(d * d * w, strata, sum)
     s2hat <- sum((mn2 - mns^2)/ns)
     c(mns[2]-mns[1],s2hat)
}

grav.boot <- boot(grav1, grav.fun, R=499, stype="w", strata=grav1[,2])
plot(grav.boot)
# now suppose we want to look at the studentized differences.
grav.z <- (grav.boot$t[,1]-grav.boot$t0[1])/sqrt(grav.boot$t[,2])
plot(grav.boot,t=grav.z,t0=0)

# In this example we look at the one of the partial correlations for the
# head dimensions in the dataset frets.
pcorr <- function( x )
{ 
#  Function to find the correlations and partial correlations between
#  the four measurements.
     v <- cor(x);
     v.d <- diag(var(x));
     iv <- solve(v);
     iv.d <- sqrt(diag(iv));
     iv <- - diag(1/iv.d) %*% iv %*% diag(1/iv.d);
     q <- NULL; 
     n <- nrow(v);
     for (i in 1:(n-1)) 
          q <- rbind( q, c(v[i,1:i],iv[i,(i+1):n]) );
     q <- rbind( q, v[n,] );
     diag(q) <- round(diag(q));
     q
}

frets.fun <- function( data, i )
{    d <- data[i,];
     v <- pcorr( d );
     c(v[1,],v[2,],v[3,],v[4,])
}
frets.boot <- boot(log(as.matrix(frets)), frets.fun, R=999)
plot(frets.boot, index=7, jack=TRUE, stinf=FALSE, useJ=FALSE)
\end{ExampleCode}
\end{Examples}