Arbitrarily re-ordering histogram columns in R

I would like to know how I can re-order the columns of a histogram in a way that makes sense to my data. This example illustrates what I'm trying to do.

I have this data in a file:

blue    low
blue    medium
blue    high
blue    high
blue    high
blue    medium
green   low
green   low
green   low
green   high
pink    low
pink    high
pink    medium
pink    low
pink    high
red     high
red     low
red     low
red     low
red     medium
red     medium
red     medium

If I run these commands:

colours <- read.table("colours.txt", sep="\t")
library(lattice)
histogram(~ V2 | V1, data=colours,  type="count")

I get pretty much what I want except that the columns in the histograms are sorted alphabet开发者_如何转开发ically, high, low, medium and I would like to have them sorted in the more natural way low, medium, high.

Thanks very much in advance for any pointers on how to accomplish this.

You just need to order your factors:

colours$V2 = factor(colours$V2, levels=c("low", "medium", "high"))
histogram(~ V2 | V1, data=colours,  type="count")