Plot

import matplotlib.pyplot as plt
import numpy as np
from scipy import interpolate

def Lag(x, i, t):
    produit = 1
    for j in range(0, len(x)):
        if i != j:
            produit *= (t - x[j])/(x[i]-x[j])
    return produit

def Interp(x, y, t):
    somme = 0
    for i in range(0, len(x)):
        somme += y[i] * Lag(x, i, t)

    return somme

x = [2 * k * np.pi / 30 for k in range(0, 31)]
y = [np.cos(x[i]) for i in range(0, 31)]

xR = np.linspace(0,2*np.pi,31)

plt.ylim((-2,2))
plt.xlim((0,2*np.pi))
for i in range(0, 31):
    yR = []
    for j in xR:
        yR.append(Interp(x, y, j))
    plt.plot(x[i], y[i], "o")
    plt.plot(xR, yR)
    plt.show()