[R] Adjusted survival curves

[Ted Beginner (RStudio)]

For adjusted survival curves I took the sample code from here: 
https://rpubs.com/daspringate/survival
and adapted for my date, but ... have a QUESTION.

library(survival)
library(survminer)
df<-read.csv("base.csv", header = TRUE, sep = ";")
head(df)
ID start stop censor sex age stage treatment
1 1 0 66 0 2 1 3 1
2 2 0 18 0 1 2 4 2
3 3 0 43 1 2 3 3 1
4 4 0 47 1 2 3 NA 2
5 5 0 26 0 1 4 3 NA

# THANKS, DAVID WINSEMIUS for remark!!! ("R regression functions allow you to use column names.")
model <- coxph( Surv(time = start,
time2 = stop,
event = censor) ~ treatment + age + sex + stage, data = df)

plot(survfit(model), 
las=1,
xscale = 1.00,
xlab = "Months after diagnosis",
ylab = "Proportion survived",
main = "Baseline Hazard Curve")

#HOW TO EXPLAIN TO R, THAT treatment, age, sex and stage ARE FACTORS, AND NOT CONTINUOUS VAR?
treat <- with (df,
data.frame (
treatment = levels(treatment),
age = rep(levels(age)[1], 2),
sex = rep(levels(sex)[1], 2),
stage = rep(levels(stage)[1], 2)))

str(treat)
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