为啥我画ROC曲线，代码都对，结果图显示不出来？大家帮帮忙看看我这程序哪有问题？

July.

library(ROCR)
logit.out<- glm(LOW~AGE+LWT+SMOKE,family=binomial(link=logit),data=Ldata)
prob<- predict(logit.out,type="response")
pred<- prediction(prob,LOW)
perf<- performance(pred,"tpr","fpr")
plot(perf）