\name{lengthKey}
\alias{lengthKey}
\title{Key for interpreting lengths in a plot}
\description{Key for interpreting lengths in a plot}
\usage{
 lengthKey(x,y,tickpos,scale)
}
\arguments{
 \item{x,y}{The position of the left end of the key in user units.}
 \item{tickpos}{The labels that will appear above the key.}
 \item{scale}{A value that will scale the length of the key.}
}
\value{nil}
\details{
 \samp{lengthKey} displays a line with tick marks and the values in
 \samp{tickpos} above those tickmarks. It is useful when line segments
 on a plot represent numeric values. Note that if the plot does not have
 a 1:1 aspect ratio, a length key is usually misleading.
}
\author{Jim Lemon}
\seealso{\link{segments}, \link{arrows}}
\examples{
 # manufacture a matrix of orientations in radians
 o<-matrix(rep(pi*seq(0.1,0.8,by=0.1),7),ncol=8,byrow=TRUE)
 m<-matrix(rnorm(56)+4,ncol=8,byrow=TRUE)
 # get an empty plot of approximately 1:1 aspect ratio
 plot(0,xlim=c(0.7,8.3),ylim=c(0.7,7.3),type="n")
 vectorField(o,m,vecspec="rad")
 # the scaling usually has to be worked out by trial and error
 lengthKey(0.3,-0.5,c(0,5,10),0.24)
}
\keyword{misc}