How to use a table in R from an imported file that has blank cells?

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programming

I'm trying to use a table I have on an external file by importing it and using it to make some graphics.

On the blank spaces in this document I put NA, but it still considering them as values...

How can I make it so these values are considered non-existing ?

My script:

datafilename <- "SGX.txt"
person.data  <- read.table(datafilename, header=TRUE, na.strings = c("NA", " "))

panel.cor <- function(x, y, digits=2, prefix="", cex.cor, ...)
{
    usr <- par("usr"); on.exit(par(usr))
    par(usr = c(0, 1, 0, 1))
    r <- abs(cor(x, y))
    txt <- format(c(r, 0.123456789), digits=digits)[1]
    txt <- paste(prefix, txt, sep="")
    if(missing(cex.cor)) cex.cor <- 0.8/strwidth(txt)
    text(0.5, 0.5, txt, cex = cex.cor * r)
}

pairs(~O3_s+NO2_s+CO_s1+CO_s2+T_ref+HR_ref+HA_ref+PP_ref+Rad_ref+VV_ref+DV_ref+P_ref, data=person.data,
      upper.panel=panel.smooth, lower.panel=panel.cor, 
      pch=20, main="SGX Scatterplot Matrix")

summary(person.data)

boxplot(person.data)

PS: I'm using R, codeproject didn't recognize the tag so I put programming.