[R] error bars on survival curve

[Murray Pung]

I am using plot(survfit(Surv(time,status) ~...) and would like to add
error bars rather than the confidence intervals. Am I able to do this
at specified times? e.g. when time = 20 & 40.


leukemia.surv <- survfit(Surv(time, status) ~ x, data = aml)
plot(leukemia.surv, lty = 2:3,xlim = c(0,50))
#can i add error bars at times 20 & 40?
legend(100, .9, c("Maintenance", "No Maintenance"), lty = 2:3)


Thanks
Murray
-- 
Murray Pung