.tr @@
.BG
.FN histbackback
.TL
Back to Back Histograms
.DN
Takes two vectors or a list with `x' and `y' components, and produces 
back to back histograms of the two datasets.
.CS
histbackback(x, y, brks=NULL, xlab=NULL, axes=T, probability=F,
             xlim=NULL, ylab='', ...)
.RA
.AG x,y
either two vectors or a list given as `x' with two components.  If the
components have names, they will be used to label the axis
(modification FEH).
.OA
.AG brks
vector of the desired breakpoints for the histograms.
.AG xlab
a vector of two character strings naming the two datasets.
.AG axes
logical flag stating whether or not to label the axes.
.AG probability
logical flag: if `TRUE', then the x-axis corresponds to the units for a
density.  If `FALSE', then the units are counts.
.AG xlim
x-axis limits.  First value must be negative, as the left histogram is
placed at negative x-values.  Second value must be positive, for the
right histogram.  To make the limits symmetric, use e.g. `ylim=c(-20,20)'.
.AG ylab
label for y-axis.  Default is no label.
.AG ...
additional graphics parameters may be given.
.RT
a list is returned invisibly with the following components:
.RC left
the counts for the dataset plotted on the left.
.RC right
the counts for the dataset plotted on the right.
.RC breaks
the breakpoints used.
.SE
a plot is produced on the current graphics device.
.SH AUTHOR
Pat Burns
.sp 0
Salomon Smith Barney
.sp 0
London
.sp 0
pburns@dorado.sbi.com
.SA
`hist', `histogram'
.EX
histbackback(rnorm(20), rnorm(30))

fool <- list(x=rnorm(40), y=rnorm(40))
histbackback(fool)
histbackback(split(age, sex))
histbackback(list(Female=agef,Male=agem), probability=T, xlim=c(-20,20))
.KW dplot
.KW hplot
.KW distribution
.WR