hexbin log scale is broken in matplotlib 1.2.0

Following code based on the hexbin demo on matplotlib site:

    import numpy as np
    import matplotlib.pyplot as plt
    np.random.seed(0)
    n = 100000
    x = np.random.standard_normal(n)
    y = 2.0 + 3.0 * x + 4.0 * np.random.standard_normal(n)
    y = np.power(2, y)
    hexbin(x,y,yscale='log')

It produces a incorrect image on 1.2.0. When reverting hexbin implementation to that of 1.1.1 (with no other changes in 1.2.0 code) it produces a correct image.

![plt111](https://f.cloud.github.com/assets/3186905/43540/0a7dd93c-568c-11e2-878e-d062a6e26ae6.png)

![plt120](https://f.cloud.github.com/assets/3186905/43541/0a9d73fa-568c-11e2-8c2f-9d873750f40a.png)

I am using Windows 64-bit Python 2.7, with matplotlib-1.2.0.win-amd64-py2.7 package and Numpy-MKL, both from http://www.lfd.uci.edu/~gohlke/pythonlibs/. Since the revert to fix was in axes.py only I believe this was not specific to the matplotlib binary.