@percusse \documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{figure}[htb]
\centering
\begin{tikzpicture}[no markers, baseline]
\begin{axis}
[
no marks,
% no markers,
% mark=none,
legend columns=-1,
legend entries={$Pe_{ax} = 1$;,$Pe_{ax} = 10$;,$Pe_{ax} = 100$;,$Pe_{ax} = 1000$},
legend to name=named3,
]
\addplot+%
[
no marks,
% no markers,
% mark=none,
smooth,
color=black,
dashed
] coordinates {(1,0) (1,2)};
\end{axis}
\end{tikzpicture}
\ref{named3}
\caption{Text.}
\end{figure}