title: ggplot change x-axis labels without changing underlying data
body: <p>I am plotting with facets and discrete data on the x-axis. How can I change the x-axis labels <strong>without</strong> changing the underlying data? I.e. instead of labels reading <code>"a","b","c","d"</code> they should read <code>"Lab 1","Lab 2","Lab 3","Lab 4"</code>.</p>
  
  <pre><code>df &lt;- data.frame(x = factor(rep(c("a","b","c","d"),4), levels = c("a","b","c","d")),
                   y = rep(seq(1,8,1),2),
                   z = c(rep("x",4),rep("y",4)),
                   facet = rep(c(rep("1",2),rep("2",2)),4))
  
  
  ggplot(NULL) +
    geom_line(data=df, aes(y=y, x=x, linetype=z, group = z, colour=z)) +
    facet_grid(~facet, scales="free_x", space="free_x") +
    scale_colour_manual(values=c("Red","Blue")) +
    scale_linetype_manual(values=c("solid", "dashed"))
  </code></pre>