title: geom_line does not generate lines using fun.y = mean as an argument
body: <p>Here is my data:</p>
  
  <pre><code>head(BCM)
          fips      scc pollutant emissions  type year
  114288 24510 10100601  PM25-PRI     6.532 POINT 1999
  114296 24510 10200601  PM25-PRI    78.880 POINT 1999
  114300 24510 10200602  PM25-PRI     0.920 POINT 1999
  114308 24510 30100699  PM25-PRI    10.376 POINT 1999
  114325 24510 30183001  PM25-PRI    10.859 POINT 1999
  114329 24510 30201599  PM25-PRI    83.025 POINT 1999
  
  table(BCM$type)
  NON-ROAD NONPOINT  ON-ROAD    POINT 
   416      142     1119      419 
  
  table(BCM$year)
  1999 2002 2005 2008 
  320  535  542  699 
  </code></pre>
  
  <p>I want to look at how the pollutant varies across time for each type. Here is my code:</p>
  
  <pre><code>ggplot(aes(x = year, y = emissions), data = BCM) +
  geom_point(stat = "summary", fun.y = mean) +
  facet_wrap(~type, scale = "free")
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/4GkaA.png" alt="with geim_point"></p>
  
  <p>What I really want to do is to connect the dots to form lines. So I thought this would work:</p>
  
  <pre><code>ggplot(aes(x = year, y = emissions), data = BCM) +
  geom_line(stat = "summary", fun.y = mean) +
  facet_wrap(~type, scale = "free")
  </code></pre>
  
  <p>But what I got is many lines of warnings and a plot with no points or lines on it:</p>
  
  <pre><code>geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?
  geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?
  geom_path: Each group consist of only one observation. Do you need to adjust the group     aesthetic?
  geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/CmEij.png" alt="with geom_line"></p>
  
  <p>I'm relatively new to <strong>R</strong>. So please somebody tell me what went wrong?</p>