How to force polyfit with second degree to a y-ntercept of 0
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I've been using the numpy.polyfit function to do some forecasting. If I put in a degree of 1, it works, but I need to do a second degree polynomial fit. In some cases it works, in other cases the plot of the prediction goes down and then goes up forever.
for example:
    import matplotlib.pyplot as plt
    from numpy import *

    x=[1,2,3,4,5,6,7,8,9,10]
    y=[100,85,72,66,52,48,39,33,29,32]
    fit = polyfit(x, y, degree) 
    fitfunction = poly1d(z4)
    to_predict=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
    
    plt.plot(to_predict,fitfunction(to_predict))
    plt.show()

After I run that, this shows up (I tried putting a picture up but stackoverflow won't let me)


I want to force it to go through zero.

How would I do that?
    
