title: error when plotting log'd array in matplotlib/scipy/numpy
body: <p>I have two arrays and I take their logs. When I do that and try to plot their scatter plot, I get this error:</p>
  
  <pre><code>  File "/Library/Python/2.6/site-packages/matplotlib-1.0.svn_r7892-py2.6-macosx-10.6-universal.egg/matplotlib/pyplot.py", line 2192, in scatter
      ret = ax.scatter(x, y, s, c, marker, cmap, norm, vmin, vmax, alpha, linewidths, faceted, verts, **kwargs)
    File "/Library/Python/2.6/site-packages/matplotlib-1.0.svn_r7892-py2.6-macosx-10.6-universal.egg/matplotlib/axes.py", line 5384, in scatter
      self.add_collection(collection)
    File "/Library/Python/2.6/site-packages/matplotlib-1.0.svn_r7892-py2.6-macosx-10.6-universal.egg/matplotlib/axes.py", line 1391, in add_collection
      self.update_datalim(collection.get_datalim(self.transData))
    File "/Library/Python/2.6/site-packages/matplotlib-1.0.svn_r7892-py2.6-macosx-10.6-universal.egg/matplotlib/collections.py", line 153, in get_datalim
      offsets = transOffset.transform_non_affine(offsets)
    File "/Library/Python/2.6/site-packages/matplotlib-1.0.svn_r7892-py2.6-macosx-10.6-universal.egg/matplotlib/transforms.py", line 1924, in transform_non_affine
      self._a.transform(points))
   File "/Library/Python/2.6/site-packages/matplotlib-1.0.svn_r7892-py2.6-macosx-10.6-universal.egg/matplotlib/transforms.py", line 1420, in transform
      return affine_transform(points, mtx)
  ValueError: Invalid vertices array.
  </code></pre>
  
  <p>the code is simply:</p>
  
  <pre><code>myarray_x = log(my_array[:, 0])
  myarray_y = log(my_array[:, 1])
  plt.scatter(myarray_x, myarray_y)
  </code></pre>
  
  <p>any idea what could be causing this?  thanks.</p>