R ggplot2 strange behaviour. It looks it's passing by reference

Question

I'm trying to copy a ggplot object and then change some properties of the new copied object as, for instance, the colour line to red.

Assume this code:

df = data.frame(cbind(x=1:10, y=1:10))
a = ggplot(df, aes(x=x, y=y)) + geom_line()
b = a

Then, if I change the colour of line of variable a

a$layers[[1]]$geom_params$colour = "red"

it also changes the colour of b

> b$layers[[1]]$geom_params$colour 
[1] "red"    # why it is not "black"?  

I wish I could have two different objects a and b with different characteristics. So, in order to do this in the correct way, I would need to call the plot again for b using b = ggplot(df, aes(xy, y=z)) + geom_line(). However, at this time in the algorithm, there is no way to know the plot command ggplot(df, aes(x=x, y=y)) + geom_line()

Do you know what's wrong with this? Is ggplot objects treated in a different manner?

Thanks!

Answer 1

The issue here is that ggplot uses the proto library to mimic OO-style objects. The proto library relies on environments to collect variables for objects. Environments are passed by reference which is why you are seeing the behavior you are (and also a reason no one would probably recommend changing the properties of a layer that way).

Anyway, adapting an example from the proto documentaiton, we can try to make a deep copy of the laters of the ggplot object. This should "disconnect" them. Here's such a helper function

duplicate.ggplot<-function(x) {
    require(proto)
    r<-x
    r$layers <- lapply(r$layers, function(x) {
        as.proto(as.list(x), parent=x)
    })
    r
}

so if we run

df = data.frame(cbind(x=1:10, y=1:10))
a = ggplot(df, aes(x=x, y=y)) + geom_line()
b = a
c = duplicate.ggplot(a)

a$layers[[1]]$geom_params$colour = "red"

then plot all three, we get

which shows we can change "c" independently from "a"