Question

I would like to make a series of plots using ggplot from multiple different dataframes. I was planning on using a list and iterating over the list as follows:

libraries <- objects() #make a list of the dataframes we want to graph
for(i in libraries) {
  # create initial plots
  x1 <- qplot(data= i, V1, reorder(V2,V3), color = V3) + coord_flip()
  x2 <- ggplot(i, aes(x=reorder(V2,V3), group=V3, color=V3)) + geom_bar() 
  x3 <- ggplot(i, aes(x=V1, group=V3, color=V3)) + coord_flip() + geom_bar()
}

however I get the error message:

Error: ggplot2 doesn't know how to deal with data of class factor

presumably because ‘libraries’ is now a character variable and not a data frame. Any one have another suggestion on how to iterate through the dataframes? I suppose I could merge them with plyr and then ggplot a subset of the data but that seems to add more work.

Answer 1

The usual way to iterate over data.frames (which are just regularly organized lists) is with lapply:

 df1 <- data.frame(date = as.Date(10*365*rbeta(100, .5, .1)),group="a")
  df2 <- data.frame(date = as.Date(10*365*rbeta(50, .1, .5)),group="b")
  df3 <- data.frame(date = as.Date(10*365*rbeta(25, 3,3)),group="c")
  dfrmL <- list(df1,df2,df3)

 lapply(dfrmL, NROW)
[[1]]
[1] 100

[[2]]
[1] 50

[[3]]
[1] 25

In the case of producing a list of ggplot-objects I would imagine that the Hadley-method would instead be to use llply, but I’m not a skilled plyr-user, so let me suggest this totally untested code template:

plts <- lapply(dfrmL, function(df) qplot(qplot(data= df, 
                                          V1, reorder(V2,V3), color = V3) + 
                                  coord_flip()
       )  
 # you may need to explicitly print() or plot() the plots as stated in the R-FAQ.    
lapply(plts, print)