roots = np.polynomial.polynomial.polyroots(poly_coeff - [99, -1, 0])
x = np.linspace(np.min(roots) - 50, np.max(roots) + 50, num=1000)
plt.plot(x, poly(x), 'r-')
plt.plot(x, 99 - x, 'b-')
for root in roots:
    plt.plot(root, 99 - root, 'ro')
Finding the intersection of a curve from polyfit