from matplotlib import pyplot as plt
X=[-3, -2, -1, 0, 1, 2, 3]
Y=[9, 4, 1, 0, 1, 0, 0]

cur_x = 4
def lagr(x, y, cur_x):
	res = 0.0
	n = len(X)
	for i in range(n):
		l_i=1.0
		for j in range(n):
			if i == j:
				continue
			l_i*=(cur_x-float(x[j]))/(x[i]-x[j])
		res+=y[i]*l_i
	return res

X_new = []
Y_new = []
step=0.001
X_start,X_stop = X[0],X[-1]
while X_start<X_stop+step:
	X_new.append(X_start)
	Y_new.append(lagr(X, Y, X_start))
	X_start+=step
plt.plot(X, Y)
plt.plot(X_new,Y_new)
plt.show()