[R] Produce multiple line graphs

[John Sorkin]

I have a data frame that contains data for multiple (seven) subjects. Each subject is represented by a new value of PID.
I would like to plot the data for all seven subjects. For each subject I want to plot a line showing CT as a function of Nit, with the dots for each subject joined. I have tried to accomplish this using the by function. I get an error message, 
Error in match.fun(panel) : 
  'xx[, "CT"]' is not a function, character or symbol
I have no idea what is causing the error, nor how to correct the error, nor how to get the dots for each point be connected by a line.


Any help would be appreciated!


PID <- c( 1 ,  1   ,  1   , 1   , 2, 2, 2, 2, 3   ,   3  ,  3   ,  3   ,  4   ,  4,  4, 4    , 5, 5, 5, 5, 6, 6, 6, 6, 7   ,  7   ,    7  , 7)
Nit <- c(NA , -9.23,-11.61,-7.88,NA,NA,NA,NA,-5.59,  0.73,-10.55, -9.13,  3.67, NA, NA,-13.26,NA,NA,NA,NA,NA,NA,NA,NA,-9.36,  5.08,  -5.73, 2.02)
CT  <- c(544,459   ,432   ,NA   ,NA,NA,NA,NA,1398 ,1287  ,1049  , NA   ,543   ,474,507,NA    ,NA,NA,NA,NA,NA,NA,NA,NA,992  ,992   ,1078   ,NA)
xx  <- data.frame(PID=PID,Nit=Nit,CT=CT)
xx
by(xx,as.factor(xx[,"PID"]),plot,xx[,"Nit"],xx[,"CT"])




John David Sorkin M.D., Ph.D.
Professor of Medicine
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology and Geriatric Medicine
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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