title: combining geom_area with factor x axis
body: <p>I have a plot with factorial x axis (not convertible in value ) and I'd like to
  plot a stacked area chart</p>
  <pre><code>set.seed(1)
  df &lt;- data.frame(x = rep(paste0(letters,formatC(1:26, width=2, flag=&quot;0&quot;)),2),
                   y = rep(runif(26)+1:26,2),
                   z = c(rep(&quot;a&quot;,26),rep(&quot;b&quot;,26)))
  
  df %&gt;% ggplot(aes(x= x, y=y)) + 
    geom_area(aes(colour = z, fill = z))
  </code></pre>
  <p>what I get is a bar chart:
  <a href="https://i.stack.imgur.com/3QIng.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3QIng.png" alt="enter image description here" /></a></p>
  <p>All the solution I have found to this problem involve a conversion of the x axis value (for example <a href="https://stackoverflow.com/questions/30343494/ggplot2-geom-area-with-factorial-x-axis">here</a>)</p>
  <p>Is there any other way to obtain a stacked chart area with factorial axis?
  How can I hide the first area (&quot;b&quot;)?</p>
  <p>Thank you very much</p>