title: Placement of axis labels at minor breaks with ggplot2
body: <p>I am using ggplot2 to do some plotting of genomic data, so the basic format is that there is a chromosome and a position along it.  I convert the positions to be on a continuous scale, then put the breaks at the boundaries of the chromosomes with:</p>
  
  <p><code>scale_x_continuous("Genome Position", breaks = c(0, cumsum(chromosome_length)))</code></p>
  
  <p>That looks great, as far as the actual plotting is concerned, but the labels are then put at the start and end of the chromosomes.  I would like them to be centered along each chromosome, at the position where the minor break is drawn by default.</p>
  
  <p>Is this possible?</p>