R help - Nov 2010 - how exactly does 'identify' work?

Hi all,

#########################################
test=data.frame(x=1:26,y=-23.5+0.45*(1:26)+rnorm(26))
rownames(test)=LETTERS[1:26]
attach(test)
#test
test.lm=lm(y~x)

plot(test.lm,2)
identify(test.lm$res,,row.names(test))
# not working

plot(x,y)
identify(x,y,row.names(test))
# works fine
identify(y,,row.names(test))
# works fine
identify(x,,row.names(test))
# not working
identify(y,,y)
# works
identify(x,,y)
# not working

#####################################

My guess is that identify take the object 'x' ( the first argument ) is
the
thing that on the y axis.

However, i have tried many many ways 
trying to get the LETTERS to be identified in the QQ-plot (plot(test.lm,2))
it never works.....

I have even tried to extract the standardized residual using library(MASS),
the 'stdres' function, and put it as the first argument in identify,
still failed...

Is there any means to achieve this?

Thanks!

casper
-- 
View this message in context:
http://r.789695.n4.nabble.com/how-exactly-does-identify-work-tp3045953p3045953.html
Sent from the R help mailing list archive at Nabble.com.

I think that your problem comes from a misunderstanding.

The general rule is that you give the plot command 2 vectors, x and y (though
you can give it the vectors separately, or together in a list or matrix).  If
you give plot only a single vector then it will use this as the y vector and use
the sequence of integers from 1 to the length of y as the x variable.  Now in
your example that matches your x exactly.

In your working examples you either give the function both x and y, or only y
and the generated sequence for x happens to match your x for this specific
example, but not in general.  For your non-working examples you give only the x
variable, which is then used as the y variable and the sequence is generated for
x, so it will only identify points along the diagonal.  It does not know where
to find your y variable.

The fix, always give both x and y (the fact that your examples worked with only
y is due to your specific example, not anything general).

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of casperyc
> Sent: Tuesday, November 16, 2010 5:03 PM
> To: r-help at r-project.org
> Subject: [R] how exactly does 'identify' work?
> 
> 
> Hi all,
> 
> #########################################
> test=data.frame(x=1:26,y=-23.5+0.45*(1:26)+rnorm(26))
> rownames(test)=LETTERS[1:26]
> attach(test)
> #test
> test.lm=lm(y~x)
> 
> plot(test.lm,2)
> identify(test.lm$res,,row.names(test))
> # not working
> 
> plot(x,y)
> identify(x,y,row.names(test))
> # works fine
> identify(y,,row.names(test))
> # works fine
> identify(x,,row.names(test))
> # not working
> identify(y,,y)
> # works
> identify(x,,y)
> # not working
> 
> #####################################
> 
> My guess is that identify take the object 'x' ( the first argument
) is
> the
> thing that on the y axis.
> 
> However, i have tried many many ways
> trying to get the LETTERS to be identified in the QQ-plot
> (plot(test.lm,2))
> it never works.....
> 
> I have even tried to extract the standardized residual using
> library(MASS),
> the 'stdres' function, and put it as the first argument in
identify,
> still failed...
> 
> Is there any means to achieve this?
> 
> Thanks!
> 
> casper
> --
> View this message in context: http://r.789695.n4.nabble.com/how-
> exactly-does-identify-work-tp3045953p3045953.html
> Sent from the R help mailing list archive at Nabble.com.
> 
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