title: Fix only the x-axis in ggplot facet_wrap
body: <p>I have been using ggplot <code>facet_wrap</code> to separate the graph into different windows by measurement category.</p>
  
  <p>I set the scales to "free" but that makes both the y and x axis scales flexible.</p>
  
  <pre><code>+ facet_wrap( ~measurementCategory, scales="free")
  </code></pre>
  
  <p>I want to let the y-axis scales be "free" as different measurement categories have different ranges.</p>
  
  <p>But I want to keep my x-axis (which is a DateTime) scale "fixed".</p>
  
  <p>What is the best way to do this?</p>