geom

ggplot(data = df.m, aes(x = month, group = 1)) + 
  geom_ribbon(data= subset(df.m, df.m$visits.raw > df.m$visits.mean), aes(ymin = pmin(visits.mean, visits.raw), ymax = pmax(visits.mean, visits.raw)),fill="#008FD5", alpha="0.2") + 
  geom_line(aes(y = visits.mean), size = 1.5, color = 'grey') +
  geom_line(aes(y = visits.raw), size = 1.5, colour = "#008FD5") + 
  facet_wrap(~year, ncol = 3) + 
  scale_x_discrete(limits = month.abb) +
  scale_y_continuous(labels=comma)