Applying and doing math on two functions read from a file using pgf

There is some expansion stuff going on (which I don't understand so won't attempt to explain it). But, here is one way to achieve the desired results:

Code:

\begin{filecontents}{data.dat}
f(x) g(x)
x    x+1
\end{filecontents}

\documentclass{article}
\usepackage{xstring,pgfplotstable,filecontents}

\begin{document}

\newcommand{\getfx}[1]{
  \pgfplotstablegetelem{#1}{f(x)}\of{data.dat}
}
\newcommand{\getgx}[1]{
  \pgfplotstablegetelem{#1}{g(x)}\of{data.dat}
}

\def\SetFeightValue{%
  \getfx{0}%
  \StrSubstitute[0]{\pgfplotsretval}{x}{8}[\feightValue]%
}

\def\SetGeightValue{%
  \getgx{0}%
  \StrSubstitute[0]{\pgfplotsretval}{x}{8}[\geightValue]%
}

\SetFeightValue
\SetGeightValue
\pgfmathsetmacro{\MyResults}{(\feightValue)/(\geightValue)}

(\feightValue)/(\geightValue)=\MyResults

\end{document}

Edit

You can do this without xstring if you have the input slightly modified. The previous code fails because pgfmath does not like \x{}. So I simply change it to (\x). (You cannot leave \x along because otherwise pgfplotstable will not see the space between \x and \x+1.)

\documentclass{article}
\usepackage{pgfplotstable,filecontents}

\begin{filecontents}{data.dat}
f(\x) g(\x) h(\x)
(\x) \x+1  f(\x)/g(\x)
\end{filecontents}

\begin{document}
{
\def\x{variable x}
\def\DeclareFunctionFromTable#1{
    \pgfplotstablegetelem{0}{#1}\of{data.dat}
    \expandafter\DeclareFunctionFromTableAux\expandafter{\pgfplotsretval}{#1}
}
\def\DeclareFunctionFromTableAux#1#2{\tikzset{declare function={#2=#1;}}}

\DeclareFunctionFromTable{f(\x)}
\DeclareFunctionFromTable{g(\x)}
\DeclareFunctionFromTable{h(\x)}

\begin{tikzpicture}
    \begin{axis}
        \addplot(\x,{f(\x)});
        \addplot(\x,{g(\x)});
        \addplot(\x,{h(\x)});
    \end{axis}
\end{tikzpicture}
\end{document}

You can do this without xstring if you have the input slightly modified.

\documentclass{article}
\usepackage{pgfplotstable,filecontents}

\begin{filecontents}{data.dat}
f(\x) g(\x) h(\x)
\x{}  \x+1  f(\x)/g(\x)
\end{filecontents}

\begin{document}

\def\x{x}
\def\DeclareFunctionFromTable#1{
    \pgfplotstablegetelem{0}{#1}\of{data.dat}
    \expandafter\DeclareFunctionFromTableAux\expandafter{\pgfplotsretval}{#1}
}
\def\DeclareFunctionFromTableAux#1#2{\pgfkeys{/pgf/declare function={#2=#1;}}}

\DeclareFunctionFromTable{f(\x)}
\DeclareFunctionFromTable{g(\x)}
\DeclareFunctionFromTable{h(\x)}

\pgfmathparse{h(8)}
\pgfmathresult

\end{document}