[R] ggplot2: proper use of facet_grid inside a function

[ONKELINX, Thierry]

Dear Bryan,

In the ggplot() function you can choose between aes() and aes_string().
In the first you need to hardwire the variable names, in the latter you
can use objects which contain the variable names. So in your case you
need aes_string().

Unfortunatly, facet_grid() works like aes() and not like aes_string().
That is why you are getting errors.

A workaround would be to add a dummy column to your data.

library(ggplot2)
data <- mpg
fac1 <- "cty"
fac2 <- "drv"
res <- "displ"
data$dummy <- data[, fac2]
ggplot(data, aes_string(x = fac1, y = res)) + geom_point() +
facet_grid(.~dummy)

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Bryan Hanson
Verzonden: vrijdag 2 oktober 2009 17:21
Aan: <R Help
Onderwerp: [R] ggplot2: proper use of facet_grid inside a function

Hello Again R Folk:

I have found items about this in the archives, but I'm still not getting
it right.  I want to use ggplot2 with facet_grid inside a function with
user specified variables, for instance:

    p <- ggplot(data, aes_string(x = fac1, y = res)) + facet_grid(. ~
fac2)

Where data, fac1, fac2 and res are arguments to the function.  I have
tried

    p <- ggplot(data, aes_string(x = fac1, y = res)) + facet_grid(. ~
as.name(fac2))

and 

    p <- ggplot(data, aes_string(x = fac1, y = res)) + facet_grid(". ~
fac2")

But all of these produce the same error:

Error in `[.data.frame`(plot$data, , setdiff(cond, names(df)), drop =
FALSE) : 
  undefined columns selected

If I hardwire the true identity of fac2 into the function, it works as
desired, so I know this is a problem of connecting the name with the
proper value.

I'm up to date on everything:

R version 2.9.2 (2009-08-24)
i386-apple-darwin8.11.1 

locale:
en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] grid      datasets  tools     utils     stats     graphics 
grDevices methods  
[9] base     

other attached packages:
 [1] Hmisc_3.6-0        ggplot2_0.8.3      reshape_0.8.3     
proto_0.3-8       
 [5] mvbutils_2.2.0     ChemoSpec_1.1      lattice_0.17-25   
mvoutlier_1.4     
 [9] plyr_0.1.8         RColorBrewer_1.0-2 chemometrics_0.4   som_0.3-4 
       
[13] robustbase_0.4-5   rpart_3.1-45       pls_2.1-0          pcaPP_1.7 
       
[17] mvtnorm_0.9-7      nnet_7.2-48        mclust_3.2        
MASS_7.2-48       
[21] lars_0.9-7         e1071_1.5-19       class_7.2-48      

loaded via a namespace (and not attached):
[1] cluster_1.12.0

Thanks for any help!  Bryan
*************
Bryan Hanson
Professor of Chemistry & Biochemistry
DePauw University, Greencastle IN USA

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