That's odd. I would think that it makes more sense to set

> the format *before* the data is plot, not after.

When normed is True, hist returns a probability density so that the

integral of the histogram equals one, assuming equally spaced bins.

See

Probability density function - Wikipedia

You should be able to verify this with

# trapezoidal integration of the probability density function

from matplotlib.mlab import trapz

pdf, bins, patches = ax.hist(...)

print trapz(bins, pdf)

JDH