How to change the order of ggplot2 facets

Question

I have a data frame "pd.long" with following structure:

> str(pd.long)
'data.frame':   144 obs. of  3 variables:
$ Site   : Factor w/ 12 levels "BED","BEU","EB",..: 8 9 10 3 11 1 6 7 5 4 ...
$ Month  : Factor w/ 12 levels "Dec","Jan","Feb",..: 1 1 1 1 1 1 1 1 1 1 ...
$ Density: int  20 20 20 20 20 20 20 20 20 20 ...

I have constructed bar-plots with this piece of code:

ggplot(pd.long, aes(x=Month, y=Density, fill=Site))+
geom_bar(stat = "identity", position = "dodge", fill = "darkgray")+
xlab("Month") + ylab("Number")+
facet_wrap(~Site, nrow = 3, scales = "free")

and the graphs look like this:

Everything was fine until I noticed that the levels of variable "Site" are not in the order of my choice. ggplot orders them alphabetically, as follows:

pd.long$Site
Levels: BED BEU EB KAD KAU KB KER KOA KOB KOO PNS RO

However, I want them to be in the same order in which they appear in the variable column i.e.

KOA KOB KOO EB  PNS BED KB  KER KAU KAD RO  BEU

Any help will highly be appreciated. Thank you

Solution

reorder the factors

pd.long$Site <- factor(pd.long$Site,levels=c("KOA","KOB","KOO","EB","PNS","BED","KB","KER","KAU","KAD","RO","BEU"))

or more general answer where the order is based on the first apperance in the Site column :

pd.long$Site <- factor(pd.long$Site,levels=unique(pd.long$Site))