title: matplotlib bar plot errbar
body: <p>in the official example,we can use bar function like this:</p>
  
  <pre><code>#!/usr/bin/env python
  # a stacked bar plot with errorbars
  import numpy as np
  import matplotlib.pyplot as plt
  
  
  N = 5
  menMeans   = (20, 35, 30, 35, 27)
  womenMeans = (25, 32, 34, 20, 25)
  menStd     = (2, 3, 4, 1, 2)
  womenStd   = (3, 5, 2, 3, 3)
  ind = np.arange(N)    # the x locations for the groups
  width = 0.35       # the width of the bars: can also be len(x) sequence
  
  p1 = plt.bar(ind, menMeans,   width, color='r', yerr=womenStd,align='center')
  p2 = plt.bar(ind, womenMeans, width, color='y',bottom=menMeans, yerr=menStd,align='center')
  
  plt.ylabel('Scores')
  plt.title('Scores by group and gender')
  plt.xticks(ind, ('G1', 'G2', 'G3', 'G4', 'G5') )
  plt.yticks(np.arange(0,81,10))
  plt.legend( (p1[0], p2[0]), ('Men', 'Women') )
  
  plt.show()
  </code></pre>
  
  <p>What I don't understand is that,
  <strong>why the womenStd is used as the menMeans's yerr,
  and the menStd is used as womenMeans's yerr.</strong></p>
  
  <p>Could you please explain that for me?
  Thanks very much.</p>