title: Shapes on line graph using stat_summary
body: <p>I'm sure the answer is very simple but at the moment it eludes me. I want to make a line graph using <code>stat_summary()</code>, with different shapes for each group (representing an experimental condition) at each x-axis tick (representing a separate time point). </p>
  
  <p>Here's the data</p>
  
  <pre><code>set.seed(124)
  ID &lt;- rep(1:12, times = 3)
  Group &lt;- rep(c("A", "B", "C"), times = 12)
  score &lt;- rnorm(36, 25, 3)
  session &lt;- rep(c("s1","s2", "s3"), each = 12)
  
  df &lt;- data.frame(ID, Group, session, score)
  </code></pre>
  
  <p>Now I can get there by making a table of means for each time point. Like so.</p>
  
  <pre><code>gMeans &lt;- aggregate(score ~ session + Group, data = df, mean)
  </code></pre>
  
  <p>And then graphing it like so.</p>
  
  <pre><code>pMeans &lt;- ggplot(data = gMeans, aes(x = session, y = score, group = Group, shape = Group)) +
    geom_line(aes(linetype = Group), size = 1) +
    geom_point(size = 5, fill = "white") +
    scale_color_hue(name = "Group", l = 30) +
    scale_shape_manual(name = "Group", values = c(23,22, 21)) +
    scale_linetype_discrete(name = "Group") +
    theme_bw() 
  
  pMeans
  </code></pre>
  
  <p>However I would like to be able to skip the step of having to make the table of means by using <code>stat_summary()</code>. I can get a similar graph with different line types, but I can't work out how to get the different shapes on each axis tick for each group. I tried the code below and many different permutations of <code>geom_point()</code> and <code>geom_line()</code>, but to no avail. How do I alter the code below to get output that looks like the output derived from the code above?</p>
  
  <pre><code>pline &lt;- ggplot(df, aes(x=session, y=score, group = Group, shape = Group)) +
                  stat_summary(fun.y="mean", geom="line", size=1.1, aes(linetype=Group, shape = Group)) +
                  scale_shape_manual(values=c(1:3)) 
  
  pline
  </code></pre>