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.
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)