title: Matplotlib Colorbar change ticks labels and locators
body: <p>I would like to change the ticks locators and labels in the colorbar of the following plot.</p>
  
  <pre><code>import pandas as pd
  import matplotlib.pyplot as plt
  from matplotlib import dates as mdates
  import numpy as np
  
  # fontdict to control style of text and labels
  font = {'family': 'serif',
          'color':  (0.33, 0.33, 0.33),
          'weight': 'normal',
          'size': 18,
          }
  
  num = 1000
  x = np.linspace(-4,4,num) + (0.5 - np.random.rand(num))
  y = np.linspace(-2,2,num) + (0.5 - np.random.rand(num))
  t = pd.date_range('1/1/2014', periods=num)
  
  # make plot with vertical (default) colorbar
  fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(6, 6))
  ax.set_title('Scatter plot', fontdict=font)
  
  # plot data
  s = ax.scatter(x = x, y = y, 
                 s=50, c=t, marker='o', 
                 cmap=plt.cm.rainbow)
  
  # plot settings
  ax.grid(True)
  ax.set_aspect('equal')
  ax.set_ylabel('Northing [cm]', fontdict=font)
  ax.set_xlabel('Easting [cm]', fontdict=font)
  
  # add colorbar
  cbar = fig.colorbar(mappable=s, ax=ax)
  cbar.set_label('Date')
  
  # change colobar ticks labels and locators
  ????
  </code></pre>
  
  <p>The colorbar illustrates the time dependency. Thus, I would like to change the ticks from their numerical values (nanoseconds?) to more sensible date format like months and year (e.g., %b%Y or %Y-%m) where the interval could be for example 3 or 6 months. Is that possible?</p>
  
  <p>I tried to play unsuccessfully with cbar.formatter, cbar.locator and mdates.</p>