title: annotation_logticks() and coord_flip() seem incompatible
body: <p>I get an error that seems to be combined by using <code>annotation_logticks()</code> and <code>coord_flip()</code> on the same plot. For instance:</p>
  
  <pre><code>ggplot(mtcars, aes(x=mpg, y=disp)) + 
    geom_line() + 
    annotation_logticks(sides="l") +
    coord_flip()
  </code></pre>
  
  <p>gives the error <code>Error in unit(yticks$y, "native") : 'x' and 'units' must have length &gt; 0</code>. <code>traceback()</code> gives results that I don't totally understand, but which seem to have something to do with assigning units.</p>
  
  <p>On the other hand, <code>annotation_logticks()</code> or <code>coord_flip()</code> <em>alone</em> doesn't cause any problem.</p>
  
  <pre><code>ggplot(mtcars, aes(x=mpg, y=disp)) + 
    geom_line() + 
    annotation_logticks(sides="l") #+
    #coord_flip()
  </code></pre>
  
  <p>works fine, as does</p>
  
  <pre><code>ggplot(mtcars, aes(x=mpg, y=disp)) + 
    geom_line() + 
    #annotation_logticks(sides="l") #+
    coord_flip()
  </code></pre>
  
  <p>I could switch the <code>x</code> and <code>y</code> mappings to avoid <code>coord_flip()</code>, but this is not ideal (I have to rewrite old plots if I want to add <code>annotation_logticks()</code> for instance).</p>