new_contents: #!/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 = (0, 0, 0, 0, 0)
  womenStd = (0, 0, 0, 0, 0)
  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=menStd)
  p2 = plt.bar(ind, womenMeans, width, color='y',
               bottom=menMeans, yerr=womenStd)
  
  plt.ylabel('Scores')
  plt.title('Scores by group and gender')
  plt.xticks(ind + width/2., ('G1', 'G2', 'G3', 'G4', 'G5'))
  plt.yticks(np.arange(0, 81, 10))
  plt.legend((p1[0], p2[0]), ('Men', 'Women'))
  
  plt.show()
subject: add simple stack bar chart