从coxph估算R中因子对照组的kaplan-meier生存曲线

我已使用以下代码在R中安装了coxph模型：

surv.a <- coxph(Surv(cum.goodp, dlq.next) ~ winter + spring + summer + strata(section) + cluster(itemcode), data = nr.sample)

【代码】surv.a秋天怎么样

spring.a <- data.frame(summer =0, spring =1, winter=0, section = c('D', 'N', 'S'))
spring.surv.a <- survfit(surv.a, newdata = spring.a, type = 'kaplan-meier')
summer.a <- data.frame(summer=1, spring=0, winter=0, section = c('D', 'N', 'S') )
summer.surv.a <- survfit(surv.a, newdata = summer.a, type = 'kaplan-meier')
winter.a <- data.frame(summer=0, spring=0, winter=1, section = c('D', 'N', 'S') )
winter.surv.a <- survfit(surv.a, newdata = winter.a, type = 'kaplan-meier')