dateformatter doesn't

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.

  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)