FAQ: How to order the (factor) variables in ggplot2

When you make a bar plot for categorical (i.e., factor) variables, probably you want to order the levels of variable in some way.

The basic idea is that making data.frame in the order you want. But this does not woks well, because the levels are reordered alphabetically. Actually, this is not a power of ggplot2, but the general behavior of factor variable.

ggplot2 uses the order of levels of factor variable to determine the order of category.
In R, there are many ways to specify the order of factors.

# sample data.
d <- data.frame(Team1=c("Cowboys", "Giants", "Eagles", "Redskins"), Win=c(20, 13, 9, 12))

# basic layer and options
p <- ggplot(d, aes(y=Win)) + opts(axis.text.x=theme_text(angle=90, hjust=1))

# default plot (left panel)
# the variables are alphabetically reordered.
p + geom_bar(aes(x=Team1), stat="identity") + opts(title="Default")

# re-order the levels in the order of appearance in the data.frame
d$Team2 <- factor(d$Team1, as.character(d$Team1))
# same as
# d$Team2 <- factor(d$Team1, c("Cowboys", "Giants", "Eagles", "Redskins"))

# plot on the re-ordered variables (Team2) (middle panel)
p + geom_bar(aes(x=Team2), data=d, stat="identity") + opts(title="Order by manual")

# re-order by variable Win
# the variables are re-orderd in the order of the win
d$Team3 <- reorder(d$Team1, d$Win)

# plot on the re-ordered variables (Team3) (right panel)
p + geom_bar(aes(x=Team3), data=d, stat="identity") + opts(title="Order by variable")

Note that re-ordering the factor is also useful to specifying the order of facet and so on.

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