obtain the max y-value of a histogram

hist returns a tuple that contains the histogram bin locations and y values. Try this:

y, x, _ = plt.hist(hdata)

print x.max()
print y.max()

Note that len(y) = len(x) - 1.

In case you also want to know the corresponding x coordinate for the beginning of that interval, following @tiago's suggestion you can add:

 x[np.where(y == y.max())]