[Rd] plot.lm: "Cook's distance" label can overplot point labels

[Prof Brian Ripley]

On Wed, 18 Feb 2009, John Fox wrote:

> Dear John,
>
>> -----Original Message-----
>> From: John Maindonald [mailto:John.Maindonald at anu.edu.au]
>> Sent: February-18-09 4:57 PM
>> To: John Fox
>> Cc: 'Martin Maechler'; r-devel at r-project.org
>> Subject: Re: [Rd] plot.lm: "Cook's distance" label can overplot point
> labels
>>
>> Dear John -
>> The title above the graph is also redundant for the first of the
>> plots; do we want to be totally consistent?  I am not sure.
>
> Why not? "A foolish consistency is the hobgoblin of little minds," but maybe
> this isn't a foolish consistency.
>
>>
>> It occurs to me that the text "Cook's distance", as well as the
>> contours, might be in red.
>
> That would provide a nice visual cue (for those who aren't colour blind).

Or using a black-and-white device.  We have not hitherto assumed a 
colour device in 'stats' graphics, and given how often they are 
printed I don't think we want to start.

As so often, it seems that what looks good is in the eye of the 
beholder.  If the two of you can agree on something that you both see 
is a definite improvement, please provide a patch and examples to try 
to persuade everyone else.  (As a Wishlist item on R-bugs, so it gets 
recorded.)

>
> Best,
> John
>
>> Regards
>> John.
>>
>> John Maindonald             email: john.maindonald at anu.edu.au
>> phone : +61 2 (6125)3473    fax  : +61 2(6125)5549
>> Centre for Mathematics & Its Applications, Room 1194,
>> John Dedman Mathematical Sciences Building (Building 27)
>> Australian National University, Canberra ACT 0200.
>>
>>
>> On 18/02/2009, at 12:27 PM, John Fox wrote:
>>
>>> Dear John,
>>>
>>> It occurs to me that the title above the graph, "Residuals vs.
>>> Leverage," is
>>> entirely redundant since the x-axis is labelled "Leverage" and the y-
>>> axis
>>> "Studentized residuals." Why not use the title above the graph for
>>> "Cook's
>>> distance countours"?
>>>
>>> Regards,
>>> John
>>>
>>>> -----Original Message-----
>>>> From: r-devel-bounces at r-project.org
> [mailto:r-devel-bounces at r-project.org
>>>> ]
>>> On
>>>> Behalf Of John Maindonald
>>>> Sent: February-17-09 5:54 PM
>>>> To: r-devel at r-project.org
>>>> Cc: Martin Maechler
>>>> Subject: [Rd] plot.lm: "Cook's distance" label can overplot point
>>>> labels
>>>>
>>>> The following code demonstrates an annoyance with plot.lm():
>>>>
>>>> library(DAAGxtras)
>>>> x11(width=3.75, height=4)
>>>> nihills.lm <- lm(log(time) ~ log(dist) + log(climb), data = nihills)
>>>> plot(nihills.lm, which=5)
>>>>
>>>> OR try the following
>>>> xy <- data.frame(x=c(3,1:5), y=c(-2, 1:5))
>>>> plot(lm(y ~ x, data=xy), which=5)
>>>>
>>>> The "Cook's distance" text overplots the label for the point with the
>>>> smallest residual.  This is an issue when the size of the plot is
>>>> much
>>>> less than the default, and the pointsize is not reduced
>>>> proportionately.
>>>>
>>>>
>>>> I suggest the following:
>>>>      xx <- hii
>>>>      xx[xx >= 1] <- NA
>>>> ## Insert new code
>>>>      fracht <- (1.25*par()$cin[2])/par()$pin[2]
>>>>      ylim[1] <- ylim[1] - diff(ylim)*max(0, fracht-0.04)
>>>> ## End insert new code
>>>>      plot(xx, rsp, xlim = c(0, max(xx, na.rm = TRUE)),
>>>>           ylim = ylim, main = main, xlab = "Leverage",
>>>>           ylab = ylab5, type = "n", ...)
>>>>
>>>> Then, about 15 lines further down, replace
>>>>        legend("bottomleft", legend = "Cook's distance",
>>>>               lty = 2, col = 2, bty = "n")
>>>>
>>>> by
>>>>        legend("bottomleft", legend = "Cook's distance",
>>>>               lty = 2, col = 2, bty = "n", y.intersp=0.5)
>>>>
>>>> If this second change is not made, then one wants fracht <-
>>>> (1.5*par()
>>>> $cin[2])/par()$pin[2]
>>>> I prefer the "Cook's distance" text to be a bit closer to the x-axis,
>>>> as it separates it more clearly from any point labels.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595