title: how to align the legend title to the middle of legend box in ggplot2?
body: <p>I want to move the legend title <code>sex</code> a little right to the horizontal center of legend box. I tried <code>theme</code> and <code>guide_legend</code> but failed. Both ways won't change the legend title position.</p>
  
  <pre><code># example data from http://www.cookbook-r.com/Graphs/Legends_(ggplot2)/
  df1 &lt;- data.frame(
      sex = factor(c("Female","Female","Male","Male")),
      time = factor(c("Lunch","Dinner","Lunch","Dinner"), levels=c("Lunch","Dinner")),
      total_bill = c(13.53, 16.81, 16.24, 17.42)
  )
  
  library(ggplot2)
  p &lt;- ggplot(data=df1, aes(x=time, y=total_bill, group=sex, shape=sex, colour=sex)) +
      geom_line() + geom_point()
  
  # no change
  p + theme(legend.title = element_text(hjust = 0.5))
  p + guides(color=guide_legend(title.hjust=0.5))
  </code></pre>
  
  <p>In addition, I am using ggplot2_2.2.0.</p>