title: ggplot2: Single smooth line for several line plots within facets
body: <p>I want to plot multiple facets with several lines each. In every facet, for the set of line plots, I would like to show a kind of smooth line representing a general trend for all those lines in a given particular facet.
  Does anybody know if it is possible? I have tried so far something like this:</p>
  
  <p><code>ggplot(data = mydata, aes(x = x, y = y, group = group)) + geom_line('GRAY') + facet_wrap(~ class) + geom_smooth('loess')</code>
  <a href="https://i.stack.imgur.com/mG8Qv.png" rel="nofollow">(see figure)</a></p>
  
  <p>However, what I observe is another line for every line plot within each facet, instead of a single line within every facet.</p>
  
  <p>Thanks in advance</p>