R: saving ggplot2 plots in a list

Question

I am writing a R code that allows users to select columns from a data and plots histograms for each of them. Hence, I am using a \'for\' loop to generate the required number

Answer 1

Instead of mapping aesthetics using aes, you might be better off using aes_string:

 for(i in 1:length(u))
 {
   probabilityHistogram <- ggplot(plotData, aes_string(x=names(plotData)[i]))
   histogramList[[i]] <-  probabilityHistogram + geom_histogram(aes(y=..density..),     binwidth=bw, colour='black', fill='skyblue') + geom_density() + scale_x_continuous(names(plotData)[i]) + opts(legend.position='none')
 }

That worked for me, at least. This avoids having to subset your data and allows you to reference the column you want to plot by quoted name.

Answer 2

You can vastly simplify your code by:

1. Using facets, rather than manually arranging multiple plots
2. Melting your data with the function melt in package reshape2
3. This means you can remove the loop

Here is a complete rewrite of your code, with no loop in sight.

data_ <- swiss
data_ <- na.omit(data_)

u <- c(2, 3, 4, 5, 6)
plotData <- data_[,u]
bw <- 5
plotType <- 'frequency'

library(ggplot2)
library(reshape2)

mdat <- melt(plotData)

if(plotType=='probability'){
  ph <- ggplot(mdat, aes(value)) +
    geom_histogram(aes(y=..density..), binwidth=bw, colour='black', fill='skyblue') + 
    geom_density() + 
    facet_wrap(~variable, scales="free")
} 

if(plotType=='frequency'){
  ph <- ggplot(mdat, aes(value)) +
    geom_histogram(aes(y=..count..), binwidth=bw, colour='black', fill='skyblue') + 
    geom_density() + 
    facet_wrap(~variable, scales="free")
}

print(ph)

The resulting graphics:

Probability:

Frequency