#' Create a KM plot
#'
#' @param x a data frame that contains followup and censored columns
#' @param var a categorical variable to split the data to sub-curves
#' @param comment An additional piece of text to write in the curve
#'
#' @return p probability that curves split by var differ significantly
#'
#' @export

kaplan.meier<-function(x,var,comment=''){
	#x should have followup and censored columns	
	surv.obj<-survival::Surv(x$followup,x$censored)
    m=base::max(x$followup)

	f<-stats::as.formula(paste('surv.obj',var,sep='~'))
	s1<-survival::survfit(f,data=x)
	#str(s1)
    tit=base::sprintf('Kaplan-Meier plot by %s',var)
    cols=base::c('red','blue')
    labels=base::c(base::sprintf('%s=true',var),base::sprintf('%s=false',var))
	#plot(s1,mark.time=TRUE,col=c('red','blue'),pch=labels,main=tit)
	graphics::plot(s1,mark.time=TRUE,col=c('red','blue'),main=tit)
	s=survival::survdiff(f,data=x)
    #str(s)
    p=stats::pchisq(s$chisq, length(s$n)-1, lower.tail = FALSE)
	 sv=base::sprintf("p=%.3f",p)
    nLab=base::sprintf('N=%d',nrow(x))
    graphics::text(x=c(0.9*m,0.3*m,0.9*m),y=c(0.2,0.1,0.3),label=c(sv,comment,nLab),cex=1.2)
    lLab <- base::gsub("x=","",base::names(s1$strata))  ## legend labels
    graphics::legend("top",legend=lLab,col=cols,lty=c(1,1),horiz=FALSE, bty='n')
    p
}