[R] Survival statistics--displaying multiple plots

[Gregory Pierce]

Hello all!

I am once again analyzing patient survival data with chronic liver disease. 

The severity of the liver disease is given by a number which is continuously
variable. I have referred to this number as "meld"--model for end stage
liver disease--which is the result of a mathematical calculation on
underlying laboratory values. So, for example, I can generate a Kaplan-Meier
plot of patients undergoing a TIPS procedure with the following:

>plot(survfit(Surv(days,status==1),subset(tips,meld<10)) 

where "tips" is my data set, "days" is the number of days alive, and meld is
the meld score.

What I would like to do is display the survival graphs of patients with
meld<10, 10<meld<20, and meld>20. I am unsure about how to go about this.

Any suggestions would be appreciated.

Greg