plt.subplot(2, 2, 2)
plt.plot(X_test, f(X_test), "b")
plt.scatter(X_train, y_train, c="b", s=20)
plt.plot(X_test, bdt_predict, "y", lw=2)
plt.title("Bagging for Trees, MSE = %.2f" % np.sum((y_test - bdt_predict) ** 2))；