Caption a tikzpicture on standalone

Question

Title says it all:

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{arrows}
\usepackage{caption}

\begin{document}

\begin{figure}
\centering
\begin{tikzpicture}
  \begin{axis}[
    title=myTtitle,
    xlabel=$x$,
    ylabel=$y$]
    \addplot coordinates {
        (10, 1) 
        (20, 2)
    };
  \end{axis}
\end{tikzpicture}
\caption{A caption.}
\end{figure}
\end{document}

Error:

LaTeX Error: Something's wrong--perhaps a missing \item.

on \begin{figure}.

Shall you change standalone to document it will work, but it will make the plot so small in comparison with the standalone version... :/

Answer 1

The same result as with John Kormylo answer you obtain with option preview of standalone package (with out use of mini pages), so see extended version of my comment:

\documentclass[preview]{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{caption}

\begin{document}
    \begin{figure}
    \centering
\begin{tikzpicture}
  \begin{axis}[
    name=border,
    title=myTtitle,
    xlabel=$x$,
    ylabel=$y$]
    \addplot coordinates {
        (10, 1)
        (20, 2)
    };
  \end{axis}
\end{tikzpicture}
\caption{A caption.}
    \end{figure}
\end{document}

Answer 2

Just for fun, I centered the axis box rather than the "axis plus labels" (assuming the left offset is larger than the right offset).

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{arrows}
\usepackage{caption}

\newlength{\offset}

\begin{document}
\begin{minipage}{5in}

\begin{figure}
\centering
\begin{tikzpicture}
  \begin{axis}[
    name=border,
    title=myTtitle,
    xlabel=$x$,
    ylabel=$y$]
    \addplot coordinates {
        (10, 1) 
        (20, 2)
    };
  \end{axis}
  \pgfextractx{\offset}{\pgfpointanchor{current bounding box}{west}}% center axis, not labels
  \path (border.east) +(-\offset,0);
\end{tikzpicture}
\caption{A caption.}
\end{figure}

\end{minipage}
\end{document}