title: Matplotlib requirements with pip install in virtualenv
body: <p>I have a requirements.txt file like this:</p>
  
  <pre><code>numpy
  matplotlib
  </code></pre>
  
  <p>When I try <code>pip install -r requirements.txt</code> inside a new virtualvenv, I get this:</p>
  
  <pre><code>REQUIRED DEPENDENCIES
  
               numpy: no
  
                      * You must install numpy 1.1 or later to build
  
                      * matplotlib.
  </code></pre>
  
  <p>If I install numpy first and matplotlib after, it works. However I'd like to keep using <code>pip install -r requirements.txt</code>. Is it possible?</p>