title: Create line graph comparing pre and post median values of two groups
body: <p>I have a dataframe with 2 cond, and 2 time points within each cond (pre and post).  I am looking to create a line graph showing the pre and post MEDIAN values from the sc column from a group of subjects on both conds.  I would also like there to be a line connecting the pre and post values on each cond.  ie. Pre ------ Post for cond #1, and Pre ----- Post for cond #2.</p>
  
  <p>Data:</p>
  
  <pre><code>id    cond    time    sc
  1      1       0      400
  2      1       0      370
  1      1       1      300
  2      1       1      302
  1      2       0      402
  2      2       0      380
  1      2       1      220
  2      2       1      203
  </code></pre>
  
  <p>This is just a sample of the data.  There are 11 subjects (id 1:11) in my data set for each condition at each time point.  This is the best code I have so far:</p>
  
  <pre><code>ggplot(data = df, aes(x = time, y = sc, group = cond, color = factor(cond))) +
  geom_line() +
  geom_point() +
  facet_wrap(~cond) +
  ylab("Switch Cost")
  </code></pre>
  
  <p>Right now this code gives me all the points from each condition (cond) and not just one point showing the median values.  Been searching online for quite a few hours today but can't find an example similar to what I'm trying to create.</p>