Pgfplotstable converting zeros to — in a knitr inline table

Question

how do I change each 0.0 to -- without having to reference each as string type or making the whole table string type? The actual table is quite long and it would take too long to do it manually. For the same reason there would be a marked improvement in readability if the table wasn't cluttered up with lots of zeros.

The final table should look something like

Flow            Bibble         Bobble        Gumph
Extant             1              2             3
Intant             --             4             5
My sainted aunt    --             6             7

using for example the MWE below,

\documentclass{article}
\usepackage{pgfplotstable}
\begin{document}
\pgfplotstabletypeset[columns/Flow/.style={string type,column type={l}},ignore chars={\^^M},col sep=&,row sep=\\]{
    <<results='asis',echo=FALSE>>=
        someStuff=read.table(textConnection("
                  Flow,   Bibble ,   Bobble ,   Gumph
                Extant,    1,           2,      3
                Intant,    0.0 ,        4,      5
       My sainted aunt,    0.0,         6,      7),header=F,sep=",")
    print(xtable(someStuff),include.rownames=F,include.colnames=F,hline.after=NULL,only.contents=T)
    @
}
\end{document}