title: cannot change font to Helvetica in Matplotlib in Python on Mac OS X 10.6
body: <p>I am trying to change the matplotlib font to helvetica, which I'd like to use in a PDF plot.  I try the following:</p>
  
  <pre><code>import matplotlib
  matplotlib.use('PDF')
  import matplotlib.pylab as plt
  from matplotlib import rc
  plt.rcParams['ps.useafm'] = True
  rc('font',**{'family':'sans-serif','sans-serif':['Helvetica']})
  plt.rcParams['pdf.fonttype'] = 42
  </code></pre>
  
  <p>This does not work -- when I run my code with --verbose-debug, I get the error:</p>
  
  <pre><code>backend WXAgg version 2.8.10.1
  /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/__init__.py:833: UserWarning:  This call to matplotlib.use() has no effect
  because the the backend has already been chosen;
  matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
  or matplotlib.backends is imported for the first time.
  findfont: Could not match :family=sans-serif:style=normal:variant=normal:weight=normal:stretch=normal:size=medium. Returning /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/ttf/Vera.ttf
  Assigning font /F1 = /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/ttf/Vera.ttf
  Embedding font /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/ttf/Vera.ttf
  Writing TrueType font
  </code></pre>
  
  <p>So apparently it cannot find Helvetica.  I am not sure why.  I have Helvetica in the afm directory of mpl-data, and when matplotlib initiates it reads it and outputs:</p>
  
  <pre><code>createFontDict: /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/afm/Helvetica.afm
  </code></pre>
  
  <p>Do I need a special .ttf Helvetica font in addition?  If so, how can I get it?  I know I have Helvetica on my system since I see it in Illustrator and many other programs.  </p>
  
  <p>I am using Enthought Python distribution as follows:</p>
  
  <pre><code>$ python
  Enthought Python Distribution -- http://www.enthought.com
  Version: 6.2-2 (32-bit)
  
  Python 2.6.5 |EPD 6.2-2 (32-bit)| (r265:79063, May 28 2010, 15:13:03) 
  [GCC 4.0.1 (Apple Inc. build 5488)] on darwin
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import matplotlib
  &gt;&gt;&gt; matplotlib.__version__
  '0.99.3'
  </code></pre>
  
  <p>Any ideas how this can be fixed?</p>
  
  <p>thanks.</p>