Let us interpolate $\cos(x)$ on uniformly spaced points in the interval $[0,2\pi]$.
from numpy import pi,cos,linspace,polyfit,polyval
from matplotlib.pyplot import figure,subplot,plot,axis,text
xmin, xmax = 0.0, 2.0*pi
fun = lambda x: cos(x)
xx = linspace(xmin,xmax,100);
ye = fun(xx);
figure(figsize(10,8))
for i in range(1,7):
N = 2*i;
subplot(3,2,i)
x = linspace(xmin,xmax,N+1);
y = fun(x);
P = polyfit(x,y,N);
yy = polyval(P,xx);
plot(x,y,'o',xx,ye,'--',xx,yy);
axis([xmin, xmax, -1.1, +1.1])
text(3.0,0.5,'N='+str(N))