new_contents: #!/usr/bin/env python
  # -*- coding: utf-8 -*-
  
  import numpy as np
  import matplotlib.pyplot as plt
  
  x = np.arange(6)
  y = x
  y_err = y / 2.
  
  plt.errorbar(x, y, yerr=y_err, fmt='-o')
  plt.show()
  
subject: Add 'matplotlib/error_bar.py'.