Continuing with the price data of BHP and VALE, look at the difference of their close prices and fit it to a polynomial of the third power:

bhp=np.loadtxt('BHP.csv', delimiter=',', usecols=(6,), unpack=True)
vale=np.loadtxt('VALE.csv', delimiter=',', usecols=(6,), unpack=True)
t = np.arange(len(bhp))
poly = np.polyfit(t, bhp - vale, 3)
print("Polynomial fit", poly)

The polynomial fit (in this example, a cubic polynomial was chosen) is as follows:

Polynomial fit [  1.11655581e-03  -5.28581762e-02   5.80684638e-01   5.79791202e+01]

The numbers you see are the coefficients of the polynomial. Extrapolate to the next value with the polyval() function and the polynomial object that we got from the fit:

print("Next value", np.polyval(poly, t[-1] + 1))

The next value we predict will be this:

Next value 57.9743076081

Ideally, the difference...