[R] Warning trying to plot -log(log(survival))

[John Sorkin]

windows xp
R 2.8.1

I am trying to plot the -log(log(survival)) to visually test the proportional hazards assumption of a Cox regression. The plot, which should give two lines (one for each treatment) gives only one line and a warning message. I would appreciate help getting two lines, and an explanation of the warning message. My problem may the that I have very few events in one of my strata, but I don't know.
Thanks,
John

fit0<-coxph(Surv(Time30,Died)~strata(Rx)+Age,data=GVHDdata)
plot(survfit(fit0),fun="cloglog")


WARNING: Warning in xy.coords (x, y, xlabel, ylabel, log)  :
 2 x values <=0 omitted from logarithmic plot



> print(survfit(fit0),fun="cloglog")
Call: survfit.coxph(object = fit0)

      n events median 0.95LCL 0.95UCL
Rx=0 31      5    Inf     Inf     Inf
Rx=1 32     15    Inf    1.17     Inf

John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)

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