title: Pip install Matplotlib error with virtualenv
body: <p>I am trying to install matplotlib in a new virtualenv. </p>
  
  <p>When I do: </p>
  
  <pre><code>pip install matplotlib
  </code></pre>
  
  <p>or</p>
  
  <pre><code>pip install http://sourceforge.net/projects/matplotlib/files/matplotlib/matplotlib-1.1.0/matplotlib-1.1.0.tar.gz
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>building 'matplotlib._png' extension
  
  gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -fPIC -  DPY_ARRAY_UNIQUE_SYMBOL=MPL_ARRAY_API -DPYCXX_ISO_CPP_LIB=1 -I/usr/local/include -I/usr/include -I. -I/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/numpy/core/include -I. -I/usr/include/python2.7 -c src/_png.cpp -o build/temp.linux-x86_64-2.7/src/_png.o
  
  src/_png.cpp:10:20: fatal error: png.h: No such file or directory
  
  compilation terminated.
  
  error: command 'gcc' failed with exit status 1
  </code></pre>
  
  <p>Anyone have an idea what is going on?</p>
  
  <p>Any help much appreciated.</p>