import numpy as np
import matplotlib.pyplot as plt
from skimage import data

# get some image
image = data.coins()
image = image[:,0:303]

# create array of radii
x,y = np.meshgrid(np.arange(image.shape[1]),np.arange(image.shape[0]))
R = np.sqrt(x**2+y**2)

# calculate the mean
f = lambda r : image[(R >= r-.5) & (R < r+.5)].mean()
r  = np.linspace(1,302,num=302)
mean = np.vectorize(f)(r)

# plot it
fig,ax=plt.subplots()
ax.plot(r,mean)
plt.show()