How to get log axes for a density plot with matplotlib?

I am trying to make a 2D density plot (from some simulation data) with
matplotlib. My x and y data are defined as the log10 of some
quantities. How can I get logarithmic axes (with log minor ticks)?

Here is an exemple of my code:

import numpy as np
import matplotlib.pyplot as plt

Data = np.genfromtxt("data") # A 2-column data file
x = np.log10(Data[:,0])
y = np.log10(Data[:,1])

xmin = x.min()
xmax = x.max()
ymin = y.min()
ymax = y.max()

fig = plt.figure()
ax = fig.add_subplot(111)

hist = ax.hexbin(x,y,bins='log', gridsize=(30,30), cmap=cm.Reds)
ax.axis([xmin, xmax, ymin, ymax])