title: ggplot2 geom_bar position failure
body: <p>I am using the <code>..count..</code> transformation in <code>geom_bar</code> and get the warning
  <em>position_stack requires non-overlapping x intervals</em> when some of my categories have few counts.</p>
  <p>This is best explained using some mock data (my data involves direction and windspeed and I retain names relating to that)</p>
  <pre><code>#make data
  set.seed(12345)
  FF=rweibull(100,1.7,1)*20  #mock speeds
  FF[FF&gt;60]=59
  dir=sample.int(10,size=100,replace=TRUE) # mock directions
  
  #group into speed classes
  FFcut=cut(FF,breaks=seq(0,60,by=20),ordered_result=TRUE,right=FALSE,drop=FALSE)
  
  # stuff into data frame &amp; plot
  df=data.frame(dir=dir,grp=FFcut)
  ggplot(data=df,aes(x=dir,y=(..count..)/sum(..count..),fill=grp)) + geom_bar()
  </code></pre>
  <p>This works fine, and the resulting plot shows the frequency of directions grouped according to speed. It is of relevance that the velocity class with the fewest counts (here &quot;[40,60)&quot;) will have 5 counts.
  <a href="https://i.stack.imgur.com/EqlLB.png" rel="noreferrer"><img src="https://i.stack.imgur.com/EqlLB.png" alt="Three categories of size 20 each" /></a></p>
  <p>However more velocity classes leads to a warning. For instance, with</p>
  <pre><code>FFcut=cut(FF,breaks=seq(0,60,by=15),ordered_result=TRUE,right=FALSE,drop=FALSE)
   
  </code></pre>
  <p>the velocity class with the fewest counts (now &quot;[45,60)&quot;) will have only 3 counts and ggplot2 will warn that</p>
  <p><em>position_stack requires non-overlapping x intervals</em></p>
  <p>and the plot will show data in this category spread out along the x axis.
  <a href="https://i.stack.imgur.com/tBHNa.png" rel="noreferrer"><img src="https://i.stack.imgur.com/tBHNa.png" alt="Four categories of size 15 each. Now the last one with three elements is not added on top of the corresponding bar" /></a>
  It seems that 5 is the minimum size for a group to have for this to work correctly.</p>
  <p>I would appreciate knowing if this is a feature or a bug in <code>stat_bin</code> (which <code>geom_bar</code> is using) or if I am simply abusing <code>geom_bar</code>.</p>
  <p>Also, any suggestions how to get around this would be appreciated.</p>
  <p>Sincerely</p>