

   CCoonnddiittiioonniinngg PPlloottss

        coplot(formula, data, given.values, panel = points, rows, columns,
               show.given = TRUE, col = par("fg"), pch = par("pch"),
               xlab = paste("Given :", a.name),
               ylab = paste("Given :", b.name),
               number = 6, overlap = 0.5, ...)
        co.intervals(x, number = 6, overlap = 0.5)

   AArrgguummeennttss::

    formula: a formula describing the form of conditioning
             plot.  A formula of the form `y ~ x | a' indicates
             that plots of `y' versus `x' should be produced
             conditional on the variable `a'.  A formula of the
             form `y ~ x| a * b' indicates that plots of `y'
             versus `x' should be produced conditional on the
             two variables `a' and `b'.

             `x' and `y' must be numeric, but `a' and `b' may
             be either numeric or factors.

       data: a data frame containing values for any variables
             in the formula.  By default the environment where
             `coplot' was called from is used.

   given.values: a value or list of two values which determine
             how the conditioning on `a' and `b' is to take
             place.

             When there is no `b' (i.e., conditioning only on
             `a'), usually this is a matrix with two columns
             each row of which gives an interval, to be condi-
             tioned on, but is can also be a single vector of
             numbers or a set of factor levels (if the variable
             being conditioned on is a factor). In this case
             (no `b'), the result of `co.intervals(..)' can be
             used directly as `given.values' argument.

      panel: a `function(x,y, col, pch, ...)'  which gives the
             action to be carried out in each panel of the dis-
             play.  The default is `points'.

       rows: the panels of the plot are laid out in a `rows' by
             `columns' array.  `rows' gives the number of rows
             in the array.

    columns: the number of columns in the panel layout array.

   show.given: logical (possibly of length 2 for 2 conditioning
             variables): should conditioning plots be shown for
             the corresponding conditioning variables (default
             `TRUE')

        col: a vector of colors to be used to plot the points.
             If too short, the values are recycled.

        pch: a vector of plotting symbols or characters.  If
             too short, the values are recycyled.

       xlab: character; label to use for the 1st conditioning
             variable.

       ylab: character; label to use for the 2nd conditioning
             variable.

     number: integer; the number of conditioning intervals,
             possibly of length 2 for x and y direction.

    overlap: numeric < 1; the fraction of overlap of the condi-
             tioning variables, possibly of length 2 for x and
             y direction.  When overlap < 0, there will be gaps
             between the data slices.

        ...: additional arguments to the panel function.

   DDeessccrriippttiioonn::

        This function produces two variants of the conditioning
        plots discussed in the reference below.

   VVaalluuee::

        `co.intervals(., number, .)' returns a (`number' x 2)
        `matrix', say `ci',  where `ci[k,]' is the `range' of
        `x' values for the `k'-th interval.

   RReeffeerreenncceess::

        Cleveland, W. S. (1993).  Visualizing Data.  New Jer-
        sey: Summit Press.

   SSeeee AAllssoo::

        `pairs', `panel.smooth', `points'.

   EExxaammpplleess::

        ## Tonga Trench Earthquakes
        data(quakes)
        coplot(long ~ lat | depth, data = quakes)
        given.depth <- co.intervals(quakes$depth, number=4, overlap=.1)
        coplot(long ~ lat | depth, data = quakes, given.values=given.depth)

        ## Conditioning on 2 variables:
        ll.dm <- long ~ lat | depth * mag
        coplot(ll.dm, data = quakes)
        coplot(ll.dm, data = quakes, number=c(4,7), show.given = c(T,F))
        coplot(ll.dm, data = quakes, number=c(3,7),
               overlap=c(-.5,.1)) # negative overlap DROPS values

        data(warpbreaks)
        ## given two factors
        coplot(breaks ~ 1:54 | wool * tension, data = warpbreaks, col = 'red')

        ## Example with empty panels:
        data(state)
        attach(data.frame(state.x77))#> don't need `data' arg. below
        coplot(Life.Exp ~ Income | Illiteracy * state.region, number = 3,
               panel=function(x,y,...) panel.smooth(x,y, span= .8, ...))

