[Rd] boxplot and bxp do not respect xlim by default (PR#9754)

[murdoch at stats.uwo.ca]

On 6/26/2007 8:16 AM, s.ellison at lgc.co.uk wrote:
> Full_Name: Steve Ellison
> Version: 2.4.1
> OS: Windows, Linux
> Submission from: (NULL) (194.73.101.157)
> 
> 
> bxp() allows specifcation of box locations with at=, but neither adjusts xlim=
> to fit at nor does it respect xlim provided explicitly.
> 
> This is because bxp() now includes explicit xlim as c(0.5, n+0.5), without
> checking for explicitly supplied xlim (or ylim if horizontal).
> 
> This also prevents simple added plots (eg if add=T, with at=(1:n)+0.5, the last
> box is partly off the plot.
> 
> The 'offending' code is in bxp:
>     if (!add) {
>         plot.new()
>         if (horizontal) 
>             plot.window(ylim = c(0.5, n + 0.5), xlim = ylim, 
>                 log = log, xaxs = pars$yaxs)
>         else plot.window(xlim = c(0.5, n + 0.5), ylim = ylim, 
>             log = log, yaxs = pars$yaxs)
>     }
> 
> Suggested fix:
>    if (!add) {
>         plot.new()
> 	bxp.limits <- if(!is.null(at)) {
>               c(at[1]-(at[2]-at[1])/2, at[n]+(at[n]-at[n-1])/2 ) 
>            } else {
>               c(0.5, n + 0.5)
>            }
>         if (horizontal) 
>             plot.window(ylim = if(is.null(pars$xlim)) bxp.limits else
> pars$xlim,
>                       xlim = ylim, log = log, xaxs = pars$yaxs)
>         else plot.window(xlim = if(is.null(pars$xlim)) bxp.limits else
> pars$xlim, 
>                       ylim = ylim, log = log, yaxs = pars$yaxs)
>     }
> 
> 
> This retains the current defaults for xlim with at unspecified but allows
> explicit specification of xlim. (which is the grouping level axis whether
> horizontal or vertical).

But it fails in a few other cases:  if the user sets the widths, this 
doesn't respect that setting; if the user specifies the location of one 
boxplot (so length(at) == 1) it fails when it tries to access at[2].

This is a somewhat tricky problem, that needs more careful thought than 
I have time for right now, so I'll leave it for someone else (or for 
myself in a less busy future, which may exist in some alternate universe).

What I'd suggest you do in the short term is simply to set up the plot 
axes the way you want before calling bxp, then call it with add=TRUE.

Duncan Murdoch

> 
> I've tested the above as far as producing a modified bxp and plotting a boxplot
> object, but have not tried calling direct from boxplot. boxplot() should,
> however, not need modification as xlim and ylim are, I believe, passed via the
> namedargs list in the bxp call.
> 
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