Changing the order of dodged bars in ggplot2 barplot

Question

I have a dataframe df.all and I'm plotting it in a bar plot with ggplot2 using the code below. I'd like to make it so that the order of the dodged bars is flipped. That is, so that the bars labeled "Singular" come before the bars labeled "Plural".

ggplot(df.all, aes(gram, V1, fill=number)) + 
    geom_bar(stat="identity", position="dodge") + 
    scale_x_discrete(labels=c("Grammatical","Ungrammatical")) +
    scale_y_continuous(formatter="percent", limits=c(0,1)) +
    facet_grid(. ~ experiment) + 
    scale_fill_hue("Attractor", breaks=c("S","P"), labels=c("Singular","Plural"))

I've tried doing levels(df.all$number) = c("S", "P") thinking that maybe ggplot uses the order of the levels to decide plotting order, but that didn't work. I'm not sure what else to try. Any ideas?

The contents of df.all, in case it's useful:

> df.all
  number gram     experiment        V1
1      S    G BERIMBAU_AGR_A 0.8133333
2      S    G BERIMBAU_AGR_B 0.8658537
3      S    U BERIMBAU_AGR_A 0.5436242
4      S    U BERIMBAU_AGR_B 0.4597701
5      P    G BERIMBAU_AGR_A 0.8580645
6      P    G BERIMBAU_AGR_B 0.8536585
7      P    U BERIMBAU_AGR_A 0.3087248
8      P    U BERIMBAU_AGR_B 0.3975904

> str(df.all)
'data.frame':   8 obs. of  4 variables:
 $ number    : Factor w/ 2 levels "S","P": 2 2 2 2 1 1 1 1
  ..- attr(*, "scores")= num [1:2(1d)] 0 -1
  .. ..- attr(*, "dimnames")=List of 1
  .. .. ..$ : chr  "P" "S"
 $ gram      : Factor w/ 2 levels "G","U": 1 1 2 2 1 1 2 2
 $ experiment: Factor w/ 4 levels "BERIMBAU_AGR_A",..: 1 4 1 4 1 4 1 4
 $ V1        : num  0.813 0.866 0.544 0.46 0.858 ...

Answer 1

In some cases I don't think this is possible:

layerCake<-data.frame(group=c(rep("normal",4),rep("tumor",4)),
                      class=factor(rep(c("exon","intron","intergenic","unmapped"),2),levels=rev(c("exon","intron","intergenic","unmapped")),ordered=TRUE),
                      fraction=c(.02,.25,.50,.23,.015,.20,.555,.23)
)
layerCake[layerCake$group=='normal',"reads"]<-130948403*layerCake[layerCake$group=='normal',"fraction"]
layerCake[layerCake$group=='tumor',"reads"]<-200948403*layerCake[layerCake$group=='tumor',"fraction"]
g<-ggplot(layerCake, aes(x=factor(group),y=reads, fill=factor(class),order = as.numeric(class)))+xlab("Group")+scale_fill_discrete(name="Anno Class",breaks=c("exon","intron","intergenic","unmapped"))

correct order in stacked:
g+geom_bar(stat="identity",position="stack")

incorrect order in dodge:

g+geom_bar(stat="identity",position="dodge")

let's try to reverse the order in ggplot:

g<-ggplot(lc, aes(x=factor(group),y=reads, fill=factor(class),order = -as.numeric(class)))+xlab("Group")+scale_fill_discrete(name="Anno Class",breaks=c("exon","intron","intergenic","unmapped"))
g+geom_bar(stat="identity",position="dodge")

no dice

let's try to reorder the data frame

lc <- with(lc, lc[order(-as.numeric(class)), ])
g<-ggplot(lc, aes(x=factor(group),y=reads, fill=factor(class),order = -as.numeric(class)))+xlab("Group")+scale_fill_discrete(name="Anno Class",breaks=c("exon","intron","intergenic","unmapped"))
g+geom_bar(stat="identity",position="dodge")

nope

Answer 2

I think df.all$number needs to be an ordered factor. Try df.all$number <- ordered(df.all$number)