graphvizで決定木を可視化した際のleafの名前を目的変数の名前にしたい

#学習
bst = xgb.train(param, dtrain)


#予測
dtest = xgb.DMatrix(test_x)
pred = bst.predict(dtest)


#精度の確認
from sklearn.metrics import accuracy_score
from sklearn.metrics import r2_score

score = accuracy_score(test_y, pred)
print('score:{0:.4f}'.format(score))

#重要度の可視化
xgb.plot_importance(bst,
                    importance_type='gain',
                    show_values=True)
plt.show()

print(test_y, pred)
print(r2_score(test_y,pred))

#決定木の可視化
import graphviz
graph = xgb.to_graphviz(bst)
graph.render(view=True,format='png')