as a new ggplot2 user, I am a bit lost with the amount of possibilities, and struggle to find on the net a simple answer to what I consider a simple problem.
I would
This is my solution. Tiny change in the ggplot function with the mapping parameter to aes_string.
library(gridExtra)
library(ggplot2)
p <- list()
for(i in 1:4){
p[[i]] <- ggplot(data=df,aes_string(x=df$x,y=df$y) +geom_bar(aes_string(x=class.names[i],fill=var.names[j])
}
do.call(grid.arrange,p)
Hope this helps!