From Martin Maechler <maechler@stat.math.ethz.ch>  Tue Aug 31 14:42:48 1999
From: Martin Maechler <maechler@stat.math.ethz.ch> (Martin Maechler)
Date: Tue, 31 Aug 1999 15:42:48 +0200
Subject: Suggestion for  qqplot() improvement
In-Reply-To: <199908311257.OAA00978@erna-w.ethz.ch> (message from Werner
 Stahel on Tue, 31 Aug 1999 14:57:21 +0200)
References: <199908311257.OAA00978@erna-w.ethz.ch>
Message-ID: <199908311342.PAA01425@sophie.ethz.ch>

>>>>> On Tue, 31 Aug 1999 14:57, Werner Stahel <stahel@stat.math.ethz.ch> said:

    WSt> Here is a suggestion.  It seems that qqplots, comparing a sample
    WSt> to a distribution other than the normal, are not explicitly
    WSt> available in S or R. I found

(in S-plus / Trellis it is, see below)

    WSt>	   qqplot(y, rt(300, df = 5))

    WSt> as an example in the help file to  qqplot .
    WSt> This is not very sensible. 

(it only tried to show that you can QQ-plot two datasets 
 of *different* length, rather than anything else)

    WSt> I would prefer

    WSt>   n <- length(y)
    WSt>   qqplot(qt((1:n)/(n+1), df=5), y, xlab='quantiles', ylab='data')

yes, for comparison with a theoretical distribution.

Maybe even
      qqplot(qt(ppoints(n), df=5), y, 
	     xlab='quantiles of t(* ,df=5)', ylab='data')

    WSt> and this could be shortened to

    WSt>   qqplot(y, distribution='t', df=5)

    WSt> with an easy extension of the function.

I found:

   Trellis in S-plus has
	    qqmath(formula, distribution = qnorm, f.value = ppoints, ...),
   for your example
        qqmath( ~ y, distribution = function(p) qt(p, df=5), ...)

which is slightly more general (also works with panels, banking to 45
deg,..) but quite a bit more complicated to use for non-sophisticated users.

What should we do / would we want for R?

Martin
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Suggestion for qqplot() improvement
Martin Maechler