make_df4hist

make_df4hist <-function(sf,bw){
  nb <- ceiling(max(sf$time)/bw)
  if(is.null(sf$strata)){
    sfdf <- data.frame(time=sf$time,survival=sf$surv,
                       stringsAsFactors = FALSE)
    df4hist <- sfdf %>% 
      mutate(time=cut(time,0:nb*bw)) %>% 
      group_by(time) %>% 
      summarise(TP=diff(range(survival))) %>% 
      mutate(density=TP/bw) %>% 
      ungroup() %>% 
      mutate(midtime=(as.integer(time)-1/2)*bw)
  }else{
    sfdf <- data.frame(time=sf$time,survival=sf$surv,
                       strata=rep(names(sf$strata),sf$strata),
                       stringsAsFactors = FALSE)
    df4hist <- sfdf %>% 
      mutate(time=cut(time,0:nb*bw)) %>% 
      group_by(strata,time) %>% 
      summarise(TP=diff(range(survival))) %>% 
      mutate(density=TP/bw) %>% 
      ungroup() %>% 
      mutate(midtime=(as.integer(time)-1/2)*bw)
    tmp <- do.call("rbind",strsplit(df4hist$strata,", |="))
    n_strata <- ncol(tmp)/2
    values=tmp[,2*1:n_strata,drop=FALSE]
    colnames(values)<-tmp[1,2*1:n_strata-1]
    df4hist <- data.frame(df4hist,values,stringsAsFactors = FALSE)
  }
  return(df4hist)
}