title: label every decade in colorbar for log imshow
body: <p>I want to plot some data in a matrix. The colors = z-values are logarithmic. My Problem is that only some of the ticks are labeled. To be precise "1e1, 1e10, ...". If those values are outside the data range there are no labels at all (but ticks). I want to have labels for every decade ie "1e1, 1e2, 1e3, ..." and also "1e-1, 1e-2, ...".
  Here comes a minimal example:</p>
  
  <pre><code>from matplotlib import pylab
  import numpy
  from matplotlib.colors import LogNorm
  
  data = numpy.loadtxt("data.txt")
  pylab.imshow(data, interpolation="nearest", origin="upper", aspect="equal", norm=LogNorm())
  cbar = pylab.colorbar()
  pylab.show()
  </code></pre>