Adding mean value to facets

Question

I have this plot, which calculates an index of abbundance of a bird species for a given winter: This graphic comes from this code (themes and scales excluded):

ggplot (a, aes (pentada,ika,colour=inv,group=inv,na.rm=T)) + geom_line()+
facet_wrap(~inv, ncol=3)+labs(title="SYLATR\n",x="",y="IKA")

and the data frame has 6 variables:

pentada / censos / yr / total / inv / ika

The thing is that I would like to add a line in each of the wintering seasons that shows the mean of the abbundance for all the years combined and I don't know how. should I append the mean as a new column after every wintering season? In this case, how should I do it?.

Thank you.

Answer 1

I'm not sure if you want the global mean, i.e. averaging over winters as well as days. If so, then shadow's solution above is probably best; something like this would also do:

#toy data
df <- data.frame(t = rep(1:100,9), pop = rnorm(900)+20, 
    year = rep(letters[1:9], 9, each = 100))

#make graph
ggplot(data = df, aes(x = t, y = pop, colour = year, na.rm=T)) + 
    geom_line() + facet_wrap(~year, ncol = 3) + 
    geom_line(aes(x=t, y = mean(pop)))

If you want the mean-over-winters-only, so that there is still a dynamic by day, I think you should probably add that to the data frame first, before calling ggplot.

#aggregate the mean population over years but not days
yearagg.df <- aggregate(data = df, pop ~ t, mean)

#make plot
ggplot(data = df, aes(x = t, y = pop, colour = year, na.rm=T)) + 
    geom_line() + 
    facet_wrap(~year, ncol = 3) + 
    geom_line(data = yearagg.df, aes(y = pop, x=t), color = 'black')

That second code snippet results in this graph:

UPDATE: You will probably have easier plotting if you put the averaged data back into your data frame so that you can plot all layers from the same data frame instead of mixing/matching data from multiple frames into one plot.

df.m <- merge(df, yearagg.df, by = 't', suffixes = c('.raw', '.mean'))
ggplot(data = df.m, aes(x = t, colour = year, na.rm=T)) + 
    geom_line(aes(y = pop.raw)) + 
    facet_wrap(~year, ncol = 3) + 
    geom_line(aes(y = pop.mean), color = 'gray')