How to draw enveloping line with a shaded area which incorporates a large number of data points?

For the figure above, how can I draw an enveloping line with a shaded area, similar to the figure below?

Replicating your example is easy because it's possible to calculate the min and max at each x and fill between them. eg.

import matplotlib.pyplot as plt
import numpy as np

#dummy data
y = [range(20) + 3 * i for i in np.random.randn(3, 20)]
x = list(range(20))

#calculate the min and max series for each x
min_ser = [min(i) for i in np.transpose(y)]
max_ser = [max(i) for i in np.transpose(y)]

#initial plot
fig, axs = plt.subplots()
axs.plot(x, x)
for s in y:
    axs.scatter(x, s)

#plot the min and max series over the top
axs.fill_between(x, min_ser, max_ser, alpha=0.2)

giving

For your displayed data, that might prove problematic because the series do not share x values in all cases. If that's the case then you need some statistical technique to smooth the series somehow. One option is to use a package like seaborn, which provides functions to handle all the details for you.