use ggplot in a function to which a column name is given

foo1 <- function(uv="u")
{
	# solution 1: do not use the data argument at all
	# 	(forces the use of qplot, could be more elegant)
	B = A[A$y<=5,]
	qplot(B$x, B$y, fill=B[[uv]], geom="tile")
}

Hi everyone, Hi ggplot users in particular,

ggplot makes it very easy to plot things given their names when you
use it interactively (and therefore can provide the names of the
columns).
        qplot(x,foo,data=A) where A has columns (x,y,foo,bar) for example

but I would like to use this from inside a function to which the name
of the column is given. I cannot find an elegant way to make this
work. Here are my attempts:

#------------------------------------------------------------

library(ggplot2)

A = data.frame(x=rep(1:10,10), y=rep(1:10,each=10), u=runif(100),
v=rnorm(100))

# goal: extract values for y<=5 and plot them, either for u or v

foo1 <- function(uv="u")
{
        # solution 1: do not use the data argument at all
        #       (forces the use of qplot, could be more elegant)
        B = A[A$y<=5,]
        qplot(B$x, B$y, fill=B[[uv]], geom="tile")
}

foo2 <- function(uv="u")
{
        # solution 2: extract and rename the colums, then use the data argument
        #       (enables ggplot but could be shorter)
        B = A[A$y<=5,c("x","y",uv)]
        names(B)[3] = "value"
        # rem: cannot use rename since rename(B,c(uv="value")) would not work
        qplot(x, y, fill=value, data=B, geom="tile")
        # or
        # ggplot(B,aes(x=x,y=y,fill=value)) + geom_tile()
}

foo3 <- function(uv="u")
{
        # solution 3: use the data argument and perform the extraction
directly in it
        #       (elegant and powerful but can't make it work)
        ggplot(A[A$y<=5,c("x","y",uv)],aes(x=x,y=y,fill=???)) + geom_tile()
        # or
        ggplot(A[A$y<=5,],aes(x=x,y=y,fill=???)) + geom_tile()
        # or ...
}

print(foo1("u"))
print(foo1("v"))
print(foo2("u"))
print(foo3("u"))

#------------------------------------------------------------

Any help in making foo3 work would be appreciated. Thanks in advance
for your expertise.

JiHO
---
http://jo.irisson.free.fr/

Hi Jiho,

The key to solving this problem is to use aes_string instead of aes.
Instead of the complicated munging that aes does to get the names of
the variables, aes_string works directly with strings, so that:

aes_string(x = "mpg", y = "wt") == aes(x = mpg, y = wt)

So your function would look like:

foo4 <- function(uv="u") {
 ggplot(A, aes_string(x = "x", y= "y", fill = uv)) + geom_tile()
}

Or

ggplot(A, aes(x=x, y=y)) + aes_string(fill=uv) + geom_tile()

Hope that helps!  (And I've made a note to better document aes_string
so you can discover after looking at aes)

great! I knew you would have thought this through. That's perfect. As
always there's the trade-off between writing code and documenting the
code already written. In this case the trade-off turned toward the
code part I guess.

Autodetection of strings by aes would be even greater but that would
prevent me to assign the actual strings "u", "x", "y" to an aes
element, which I don't see as a problem for non text related functions
though...