title: AttributeError: 'numpy.ndarray' object has no attribute '_hold'
body: <p>I am using numpy and matplotlib in Python3.<br>
  The following code is causing the error:   </p>
  
  <pre><code>import matplotlib
  from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
  from matplotlib.figure import Figure
  from matplotlib.axes import Subplot as plt
  from matplotlib import pylab
  
  
   a=[1,1]
   b=[1,1]
   fsam = 48000         
   w, h = freqz(b, a, worN=2000)
   plt.plot(((fsam-8000) * 0.5 / np.pi) * w, abs(h), label=" ")   
  </code></pre>
  
  <p>The actual error with matplotlib 1.3.x:   </p>
  
  <pre><code> File "/usr/local/lib/python3.2/dist-packages/matplotlib-1.3.x-py3.2-linux-x86_64.egg/matplotlib/axes.py", line 4119, in plot   
       if not self._hold:   
  AttributeError: 'numpy.ndarray' object has no attribute '_hold'   
  </code></pre>
  
  <p>The actual error with matplotlib 1.2.0:   </p>
  
  <pre><code> Traceback (most recent call last):   
     File "/home/christoph/audio_measurement/AudioTools/AudioTools.py", line 222, in &lt;module&gt;   
         main()   
       File "/home/christoph/audio_measurement/AudioTools/AudioTools.py", line 216, in main   
         form = AppForm()   
       File "/home/christoph/audio_measurement/AudioTools/AudioTools.py", line 39, in __init__   
         self.on_draw()   
       File "/home/christoph/audio_measurement/AudioTools/AudioTools.py", line 80, in on_draw   
         self.transfer = Transfer(self.canvas)   
       File "/home/christoph/audio_measurement/AudioTools/Transfer.py", line 42, in __init__   
         plt.plot(((fsam-8000) * 0.5 / np.pi) * w, abs(h), label=" ")   
       File "/usr/local/lib/python3.2/dist-packages/matplotlib/axes.py", line 3995, in plot   
         if not self._hold: self.cla()   
  AttributeError: 'numpy.ndarray' object has no attribute '_hold'   
  </code></pre>
  
  <p>Transfer is the class, which plots onto the canvas.   </p>
  
  <p>I had a look at the length of the coefficients a and b, but they did not effect the result.<br>
  I could not find anything on that. Does anyone know whats going wrong?</p>