title: matplotlib.pyplot.ticklabel_format has no effect on the figure
body: <p>as the title suggests, this is a straightforward question: ticklabel_format simply has no effect whatsoever on my figure.
  here's the script:</p>
  
  <pre><code>import sys
  import math
  import yaml
  import numpy as np
  import matplotlib.pyplot as plt
  dwarf  = sys.argv[1]
  pts = np.empty([100,100])
  fig = plt.figure()
  fig.suptitle('J value - %s'%dwarf,fontsize=18)
  m = plt.imshow(pts,cmap='rainbow',extent=[-2,2,5,9])
  plt.xlabel(r'$r_s [kpc]$',fontsize=18)
  plt.ylabel(r'$\rho_s [M_{sun} kpc^{-3}]$',fontsize=18)
  plt.ticklabel_format(style='sci',axis='x',scilimits=(-2,2))
  plt.ticklabel_format(style='sci',axis='y',scilimits=(5,9))
  plt.grid()
  cx = plt.colorbar(m,pad=0)
  cx.set_label(r'$log_{10}(J(\rho_s,r_s))$',fontsize=18)
  plt.savefig('output/gridJ_%s.png'%dwarf,dpi=100,format='png')
  plt.show()
  </code></pre>
  
  <p>on the produced plot, the ticks on the axes are simply the values dictated by extent kwarg in plt.imshow and not the nice scientific notation 10**n I would like it to have.
  Any idea why it's misbehaving? Thank you</p>