title: arrow in plot matplotlib.pyplot
body: <p>I am unable to get the arrow to display like I want it.
  Here is an example of what I am trying to do:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  
  b = np.arange(5)*-1E-4
  a = np.arange(5)
  
  fig, ax = plt.subplots()
  ax.plot(a,b, linewidth=3, color="k") 
  plt.arrow(1,-0.00010,0,-0.00005, shape='full', lw=3, length_includes_head=True, head_width=.01)
  plt.show()
  </code></pre>
  
  <p>As far as I understand, this should produce an arrow starting at (1,-0.00010) and ending at (1,-0.00015)
  But the result is a much longer line, no longer looking like an arrow, and not starting and stopping at the right points.</p>
  
  <p><a href="https://i.stack.imgur.com/3ulGD.png" rel="noreferrer"><img src="https://i.stack.imgur.com/3ulGD.png" alt="enter image description here"></a></p>