plt.subplot(2, 2, 4)
plt.plot(X_test, f(X_test), "b")
plt.scatter(X_train, y_train, c="b", s=20)
plt.plot(X_test, et_predict, "purple", lw=2)
plt.title("Extra Trees, MSE = %.2f" % np.sum((y_test - et_predict) ** 2))；