I had previously written about Image Histogram here:
A cumulative histogram sums the number of pixels having the current value or smaller than the current value.
The following function computes the cumulative histogram
"""The function takes as input a histogram [np.array] and returns the cumulative histogram [np.array]"""
cum_hist = np.zeros((256,1))
for i in range(256):
cum_hist[i] = sum(hist[:i+1])