Monthplot with 3 Series

gistfile1.r

require(ggplot2)
require(lubridate)
# df is the data frame on the page http://goo.gl/MvG02 in the answer by Andrie
# this solution modifies his approach to allow multiple time series to be 
# incorporated into the month plot

df = transform(df, values2 = values + 100, values3 = values + 200)
dfm = melt(df, id = 'dates')
dfm$month <- factor(month(dfm$dates), levels=1:12, labels=month.abb, ordered=TRUE)
dfm$year  <- year(dfm$dates)


hline.data <- ddply(dfm, .(month, variable), summarize, avgvalue=mean(value))

p0 = ggplot(dfm, aes(x = year, y = value, colour = variable)) + 
  geom_line() +
  geom_hline(data = hline.data, aes(yintercept = avgvalue, colour = variable), size=2) + 
  facet_grid(. ~ month) +
  opts(axis.text.x = theme_blank()) +
  xlab("")

print(p0)