Understand colors in R survfit.plot function for a coxph model

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I'm trying to create a set of univariate coxph models, with categorical explanatory variables and, for each model, I would like to comapre the survival functions by level of variable. For example I'm creating:

library(survival)

set.seed(4321)
data <- data.frame(cbind(
  sample(0:1, 40, replace=TRUE), # event
  sample(0:24, 40, replace=TRUE), # time
  sample(1:2, 40, replace=TRUE), # Diam
  sample(3:4, 40, replace=TRUE) # N
  ))

colnames(data) <- c("event","time","Diam","N")

data$Diam <- as.factor(data$Diam)
data$N <- as.factor(data$N)

model1 <- coxph(Surv(time, event) ~ Diam, data = data)
model2 <- coxph(Surv(time, event) ~ N, data = data)

where Diam and N are have two levels. I prepared this code for plottiing the survival functions:

plot(survfit(model1, newdata = data), fun = "s",
     conf.int = TRUE,
     col = 2:4,
     xlab = "Months",
     ylab = "Proportion without event",
     main = "Survival curve by Diam")
legend("bottomleft", 
       legend = levels(data$Diam),
       lty = 1, col = c(2,4))

plot(survfit(model2, newdata = data), fun = "s",
     conf.int = TRUE,
     col = 2:4,
     xlab = "Months",
     ylab = "Proportion without event",
     main = "Survival curve by N")
legend("bottomleft", 
       legend = levels(data$N),
       lty = 1, col = c(2,4))

and this is the output: First variable Second variable

I don't understand:

  1. Why in the second chart the colours are different?
  2. How to match colours in the chart and in the legend.

Any help would be very appreciated! Thank you in advance, Francesca

cox-regression survival-analysis plot r
1 Answers

I found strange that you put three colours when drawing the plot and only two in the legend. I managed to draw it correctly with this

plot(survfit(model1, newdata = data), fun = "s",
     conf.int = TRUE,
     col = c(1,2,2),
     xlab = "Months",
     ylab = "Proportion without event",
     main = "Survival curve by Diam")
legend("bottomleft", 
       legend = levels(data$Diam),
       lty = 1, col = levels(data$Diam))

plot(survfit(model2, newdata = data), fun = "s",
     conf.int = TRUE,
     col = levels(data$N),
     xlab = "Months",
     ylab = "Proportion without event",
     main = "Survival curve by N")
legend("bottomleft", 
       legend = levels(data$N),
       lty = 1, col = levels(data$N))

But note that I kept three colours in the first plot and only two in the second. I believe the problem is with the model or the sampling.

Also, using the a different seed (1, for example) you dont't have any problems with the colours.

set.seed(1)
data <- data.frame(cbind(
  sample(0:1, 40, replace=TRUE), # event
  sample(0:24, 40, replace=TRUE), # time
  sample(1:2, 40, replace=TRUE), # Diam
  sample(3:4, 40, replace=TRUE) # N
))

colnames(data) <- c("event","time","Diam","N")

data$Diam <- as.factor(data$Diam)
data$N <- as.factor(data$N)

model1 <- coxph(Surv(time, event) ~ Diam, data = data)
model2 <- coxph(Surv(time, event) ~ N, data = data)
plot(survfit(model1, newdata = data), fun = "s",
     conf.int = TRUE,
     col = levels(data$Diam),
     xlab = "Months",
     ylab = "Proportion without event",
     main = "Survival curve by Diam")
legend("bottomleft", 
       legend = levels(data$Diam),
       lty = 1, col = levels(data$Diam))

plot(survfit(model2, newdata = data), fun = "s",
     conf.int = TRUE,
     col = levels(data$N),
     xlab = "Months",
     ylab = "Proportion without event",
     main = "Survival curve by N")
legend("bottomleft", 
       legend = levels(data$N),
       lty = 1, col = levels(data$N))

cheers,

user_107768830
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