title: matplotlib contour plot with lognorm - colorbar levels
body: <p>I am trying to make a contour plot with defined levels and log norm. Below is an example:</p>
  
  <pre class="lang-py prettyprint-override"><code>import matplotlib.pyplot as plt
  import numpy as np
  from matplotlib.colors import LogNorm
  delta = 0.025
  
  x = y = np.arange(0, 3.01, delta)
  X, Y = np.meshgrid(x, y)
  Z1 = plt.mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
  Z2 = plt.mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
  Z = 10 * (Z1* Z2)
  
  fig=plt.figure()
  ax1 = fig.add_subplot(111)
  lvls = np.logspace(-4,0,20)
  CF = ax1.contourf(X,Y,Z,
           norm = LogNorm(),
           levels = lvls
          )
  CS = ax1.contour(X,Y,Z,
           norm = LogNorm(),
           colors = 'k',
           levels = lvls
          )
  cbar = plt.colorbar(CF, ticks=lvls, format='%.4f')
  plt.show()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/WJJcd.png" alt="enter image description here">
  My questions is: <br/>
  The levels should be written in the format: '1x10^-4', '1.6x10^-4', ... How do i do this, without specifying each level manually? <br/></p>
  
  <p>I am using python 2.7.3 with matplotlib 1.1.1 on Windows 7.</p>