title: how to use pyplot.arrow or patches.Arrow in matplotlib?
body: <p>If I wanted to draw arrows instead of axis lines using one of the arrow/Arrow functions, how do I get the line/arrowhead to show up? Example below:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import matplotlib.patches as pts
  
  plt.ion()
  ax = plt.axes()
  ax.set(xlim=[0,1],ylim=[0,1],autoscale_on=False,clip_on=False)
  ax.set(frame_on=False,xticks=[],yticks=[])
  pts.Arrow(0,0,1,0,width=0.05)
  plt.arrow(0,0,0,1,shape='right',lw=3,fill=True)
  ax.text(1,0,'functionalization',
          verticalalignment='top',
          horizontalalignment='center')
  ax.text(0,1,'void space/structure')
  plt.draw()
  </code></pre>
  
  <p>pts.Arrow draws nothing, and plt.arrow draws a line but no arrowhead.</p>