Matplotlib 1.5.1 POWER6 AIX cluster (built from source)

The following code generates a figure with broken axis tick values as values involve numbers with exponents. X-axis should be 10^0 ~ 10^5 and Y-axis should be 10^0~10^10. Basically horizontal spacing appears to be ignored. Has anyone seen this before?

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import numpy as np
x=np.arange(100000)+1.
y=x**2
plt.figure()
plt.loglog(x,y)
plt.savefig('testlog.png')