[R] how exactly does 'identify' work?

[Greg Snow]

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
> --
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> exactly-does-identify-work-tp3045953p3045953.html
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