title: Add legend to pyplot.errorbar
body: <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  # example data
  x = np.arange(0.1, 4, 0.5)
  y = np.exp(-x)
  yerr = 0.1*np.random.rand(8)
  
  fig, ax = plt.subplots()
  ax.errorbar(x, y, linestyle='none', marker='*', yerr=yerr)
  plt.show()
  </code></pre>
  
  <p>Hi, everyone! The goal is to add legend to the chart. y and yerr are labelled as 'mean' and 'std.Dev', respectively. </p>