I know I’m late to the party. But, there is a simpler way if you just want the cdf for your plot and not for future calculations:
plt.hist(put_data_here, normed=True, cumulative=True, label="CDF",
histtype="step", alpha=0.8, color="k")
As an example,
plt.hist(dataset, bins=bins, normed=True, cumulative=True, label="CDF DATA",
histtype="step", alpha=0.55, color="purple")
# bins and (lognormal / normal) datasets are pre-defined
EDIT: This example from the matplotlib docs may be more helpful.