\documentclass{article}
\usepackage{fullpage}
\usepackage{txfonts}
\usepackage{pgfplots}

% \usepgfplotslibrary{external}
% \tikzset{external/force remake}
% \tikzexternalize

\pgfplotsset{
  compat=1.5.1, % most current version as of writing (27 Jan 2012)
  % compat=newest, % select to always use latest version (may cause minor appearance changes)
  width=\textwidth,
}
\begin{document}
\begin{tikzpicture}
  \begin{loglogaxis}[
    xlabel={Observed Intensity, \(100 \times I_{\lambda}/I(\mathrm{H}\beta)\)}, 
    ylabel={Model Intensity, \(100 \times I_{\lambda}/I(\mathrm{H}\beta)\)},
    log ticks with fixed point,
    legend columns=2,
    legend pos=north west,
    transpose legend,
    ]
    \addplot [
    scatter/classes={
      Ne3={mark=square*,blue},%
      S2={mark=triangle*,red},% 
      S3={mark=square*,red},% 
      C2={mark=triangle*,black},% 
      O1={mark=*,orange},% 
      O3={mark=square*,orange},%
      He1={mark=*,green},%
      Fe3={mark=square*,yellow},%
      H1={mark=*,brown},%
      N2={mark=triangle*,cyan},%
      Ar3={mark=square*,magenta}%
    },
    only marks, scatter, 
    scatter src=explicit symbolic,
    point meta=explicit symbolic,
    nodes near coords*={\tiny\pgfmathprintnumber[set thousands separator={}]\LAMBDA\,\AA},
    visualization depends on={\thisrow{lambda} \as \LAMBDA},
    error bars/.cd,
    x dir=both,
    x explicit,
    y dir=none,
    ] table [
    x=Observed, 
    y=Model, 
    x error=Error,
    meta=El
    ] {ratios-177341.dat};
    \label[Ne3]{marker:Ne3}

    \legend{
      Ne3,
      S2,
      S3,
      C2,
      O1,
      O3,
      He1,
      Fe3,
      H1,
      N2,
      Ar3
    }
    \addplot [gray,domain=0.2:300] {x};
  \end{loglogaxis}
\end{tikzpicture}

% The marker for [Ne III] is \ref{marker:Ne3}

\end{document}