ggplot2: apply customization from variable

Question

I'm new to R and ggplot2. How would I go about applying a customization to multiple plots? I thought I could just store the customization in a variable but this does not work:

# customization <- theme_bw() + xlab("time")
x <- 1:20
y <- dnorm(x, 10, 5)
y2 <- dnorm(x, 10, 2)
p1 <- ggplot() + geom_line(aes(x,y)) # + customization
p2 <- ggplot() + geom_line(aes(x,y2)) # + customization

Now I would like to apply customization to both plots without copy/pasting the two additional settings.

Answer 1

You can put the theme elements in one object and the labels in another object and then combine them with +. This isn't quite as succinct as what you were probably hoping for, but maybe it gets you part of the way there. If you have more than one label and several theme elements, it will save you some typing. For example:

x <- rep(1:20,2)
y <- c(dnorm(x[1:20], 10, 5), dnorm(x[21:40], 20, 5))
group = factor(rep(c("A","B"),each=20))
dat=data.frame(x,y,group)

opt <- theme(title=element_text(size=18, colour="green"),
             axis.text=element_text(size=13, colour="black"),
             axis.title=element_text(size=15, colour="blue"),
             legend.title=element_text(colour="black")) 
lab <- labs(x="Time", y="Value", colour="Group", title="Plot Title")

ggplot(dat) + geom_line(aes(x,y, colour=group)) + opt + lab

UPDATE: Per @ Baptiste's comment, you can combine the theme and labs elements in a single list object:

custom <- list(opt, lab)

ggplot(dat) + geom_line(aes(x,y, colour=group)) + custom