CODEWITHSUNDEEP
rggplot2
fabb
fabb

Reputation: 11745

Local Variables Within aes

I'm trying to use a local variable in aes when I plot with ggplot. This is my problem boiled down to the essence:

xy <- data.frame(x=1:10,y=1:10)

plotfunc <- function(Data,YMul=2){
    ggplot(Data,aes(x=x,y=y*YMul))+geom_line()
}

plotfunc(xy)

This results in the following error:

Error in eval(expr, envir, enclos) : object 'YMul' not found

It seems as if I cannot use local variables (or function arguments) in aes. Could it be that it occurrs due to the content of aes being executed later when the local variable is out of scope? How can I avoid this problem (other than not using the local variable within aes)?

Upvotes: 47

Views: 11472

Answers (6)

Metamorphic
Metamorphic

Reputation: 781

I am using ggplot2, and your example seems to work fine with the current version.

However, it is easy to come up with variants which still create trouble. I was myself confused by similar behavior, and that's how I found this post (top Google result for "ggplot how to evaluate variables when passed"). For example, if we move ggplot out of plotfunc:

xy <- data.frame(x=1:10,y=1:10)

plotfunc <- function(Data,YMul=2){
  geom_line(aes(x=x,y=y*YMul))
}

ggplot(xy)+plotfunc(xy)
# Error in eval(expr, envir, enclos) : object 'YMul' not found

In the above variant, "capturing the local environment" is not a solution because ggplot is not called from within the function, and only ggplot has the "environment=" argument.

But there is now a family of functions "aes_", "aes_string", "aes_q" which are like "aes" but capture local variables. If we use "aes_" in the above, we still get an error because now it doesn't know about "x". But it is easy to refer to the data directly, which solves the problem:

plotfunc <- function(Data,YMul=2){
  geom_line(aes_(x=Data$x,y=Data$y*YMul))
}
ggplot(xy)+plotfunc(xy)
# works

Upvotes: 4

BBrill
BBrill

Reputation: 1942

Have you looked at the solution given by @wch (W. Chang)?

https://github.com/hadley/ggplot2/issues/743

I think it is the better one

essentially is like that of @baptiste but include the reference to the environment directly in the call to ggplot

I report it here

g <- function() {
  foo3 <- 4
  ggplot(mtcars, aes(x = wt + foo3, y = mpg),
         environment = environment()) +
    geom_point()
}

g()
# Works

Upvotes: 1

baptiste
baptiste

Reputation: 77096

I would capture the local environment,

xy <- data.frame(x=1:10,y=1:10)

plotfunc <- function(Data, YMul = 2){
    .e <- environment()
    ggplot(Data, aes(x = x, y = y*YMul), environment = .e) + geom_line()
}

plotfunc(xy)

Upvotes: 39

jthetzel
jthetzel

Reputation: 3623

ggplot()'s aes expects YMul to be a variable within the data data frame. Try including YMull there instead:

Thanks to @Justin: ggplot()'s aes seems to look forYMul in the data data frame first, and if not found, then in the global environment. I like to add such variables to the data frame, as follows, as it makes sense to me conceptually. I also don't have to worry about changes to global variables having unexpected consequences to functions. But all of the other answers are also correct. So, use whichever suits you.

require("ggplot2")
xy <- data.frame(x = 1:10, y = 1:10)
xy <- cbind(xy, YMul = 2)

ggplot(xy, aes(x = x, y = y * YMul)) + geom_line()

Or, if you want the function in your example:

plotfunc <- function(Data, YMul = 2)
{
    ggplot(cbind(Data, YMul), aes(x = x, y = y * YMul)) + geom_line()
}

plotfunc(xy)

Upvotes: 5

Josh O&#39;Brien
Josh O&#39;Brien

Reputation: 162311

Here's an alternative that allows you to pass in any value through the YMul argument without having to add it to the Data data.frame or to the global environment:

plotfunc <- function(Data, YMul = 2){
    eval(substitute(
        expr = {
            ggplot(Data,aes(x=x,y=y*YMul)) + geom_line()
        }, 
        env = list(YMul=YMul)))
    }

plotfunc(xy, YMul=100)

To see how this works, try out the following line in isolation:

substitute({ggplot(Data, aes(x=x, y=y*YMul))}, list(YMul=100))

Upvotes: 10

Justin
Justin

Reputation: 43245

If you execute your code outside of the function it works. And if you execute the code within the function with YMul defined globally, it works. I don't fully understand the inner workings of ggplot but this works...

YMul <- 2

plotfunc <- function(Data){
    ggplot(Data,aes(x=x,y=y*YMul))+geom_line()
}

plotfunc(xy)

Upvotes: 0

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