import matplotlib.pyplot as plt
import numpy as np
N=30
t=np.linspace(0,2*np.pi,N)
x=t
y=np.cos(x)+0.3*x+0.5*(0.5-np.random.random(N))
plt.plot(x,y,"o")

Y=np.polyfit(x,y,2)
plt.plot(x,np.polyval(Y,x),'b-')
plt.show()

xx = [1, 2, 5]
yy = [2, 5, 3]
plt.plot(xx,yy,"ro")
YY=np.polyfit(xx,yy,2)
N=len(xx)
XX=np.linspace(xx[0],xx[N-1],20*N)
plt.plot(XX,np.polyval(YY,XX),'b-')