#--Load previously saved data:
path <- "http://www.sr.bham.ac.uk/~ajrs/R/datasets"
a <- load(url(paste(path,"middayweather.RData",sep="/")))
close(url(paste(path,"middayweather.RData",sep="/")))

#--Load extra library:
## if not already installed, then run:
# install.packages("ggplot2")
require(ggplot2)

#--Calculate month from date & create factor:
middayweather$month <- with(middayweather, factor(as.POSIXlt(Date)$mon, label=month.abb))

#--Calculate year from date:
middayweather$year <- with(middayweather, 1900 + as.POSIXlt(Date)$year)

#--Only use complete years:
middayweather <- subset(middayweather, year %in% 2007:2009)

#-----Plot data as boxplot summary for each month:
theme_set(theme_gray(base_size = 11))
p <- ggplot(data=middayweather, aes(month, T.out)) + 
 geom_boxplot() + 
 facet_wrap( ~ year, nrow=1) + 
 scale_x_discrete(breaks=month.abb[c(1, 4, 7, 10)]) +
 xlab("") + 
 ylab(as.expression(expression( paste("Temperature (", degree,"C)") )))

#--Save plot to pdf:
ggsave(p, file="temperature_monthly_boxplot.pdf", width=9, height=3.5)