Center caption in standalone documentclass

Question

I would like to know how I can go about automatically centering the plots subcaptions in the standalone environment. This is what I would like:

Here is my code:

\documentclass[varwidth]{standalone}
\usepackage{pgfplots,pgfplotstable, booktabs}
\usepackage{graphicx}
\usepackage{float}
\usepackage{subfig}
\usepackage{xcolor}
\definecolor{ocre}{RGB}{0,173,239} %  mybluei Blue color
\usetikzlibrary{pgfplots.groupplots, matrix, backgrounds}
\pgfplotsset{%
    compat=1.12,
    every tick label/.append style={font=\scriptsize},
    every axis plot/.append style={line width=0.8pt},
    minor grid style={dashed,red},
    major grid style={dotted,green!50!black},
    }

\captionsetup[subfigure]{labelfont={color=ocre,scriptsize,it,bf},textfont={color=ocre,scriptsize,it,bf},labelformat=parens,labelsep=space}

\begin{document}
\begin{figure}[H]
\begin{center}
\begin{tikzpicture}
\setcaptionsubtype
\begin{groupplot}[%
            ,group style={%
                ,group name=my plots
                ,group size=1 by 2
                ,vertical sep=2cm,
                ,horizontal sep = 2cm,
                ,ylabels at=edge left
            }
            ,width=10cm
            ,height=5cm
            ,try min ticks=5
            ,xlabel={\bfseries{\emph{\footnotesize{Time (ms)}}}}
            ,ylabel={\bfseries{\emph{\footnotesize{Temperature (Deg C)}}}}
            ,grid=both
            ,every major grid/.style={gray, opacity=0.5}
            ]
\nextgroupplot%
\addplot[cyan]{rand};\label{plots:ltone}

\nextgroupplot%
\addplot[cyan]{rand};\label{plots:lttwo}

\addplot [smooth,red,thick]{rand};

\end{groupplot}

\node[text width=.5\linewidth,align=center,anchor=south] at (my plots c1r1.north) {\caption[]{Lock Time Response\label{subplot:ltone}}};
\node[text width=.5\linewidth,align=center,anchor=south] at (my plots c1r2.north) {\caption[]{Zoomed-In Lock Time Response\label{subplot:lttwo}}};

\end{tikzpicture}
%\caption[]{This is a test Plot.}
\end{center}
\end{figure}
\end{document}

Answer 1

As you can see by adding draw to the node options, the node containing the caption is positioned correctly, but the alignment of the caption inside the node is wrong.

The caption alignment can be changed by the justification key in \captionsetup, i.e. \captionsetup{justification=centering}. Presumably you don't want all the captions in the document centered, but by adding \captionsetup{justification=centering} right after \setcaptionsubtype it will be local to the tikzpicture (as mentioned by Axel Sommerfeldt in a comment).

One additional note: Neither \bfseries nor \footnotesize are commands that take an argument, the way \emph is used. They are both switches that affect the following text, and as such should be used as {\bfseries\footnotesize text}

Complete code and output:

\documentclass[varwidth,border=4mm]{standalone}
\usepackage{pgfplots}
\usepackage{float}
\usepackage{subfig}
\definecolor{ocre}{RGB}{0,173,239} %  mybluei Blue color
\usetikzlibrary{pgfplots.groupplots, matrix, backgrounds}
\pgfplotsset{%
    compat=1.12,
    every tick label/.append style={font=\scriptsize},
    every axis plot/.append style={line width=0.8pt},
    minor grid style={dashed,red},
    major grid style={dotted,green!50!black},
    }

\captionsetup[subfigure]{labelfont={color=ocre,scriptsize,it,bf},textfont={color=ocre,scriptsize,it,bf},labelformat=parens,labelsep=space}

\begin{document}
\begin{figure}[H]
\centering
\begin{tikzpicture}
\setcaptionsubtype
\captionsetup{justification=centering} % <-- added
\begin{groupplot}[%
            ,group style={%
                ,group name=my plots
                ,group size=1 by 2
                ,vertical sep=2cm,
                ,horizontal sep = 2cm,
                ,ylabels at=edge left
            }
            ,width=10cm
            ,height=5cm
            ,try min ticks=5
            ,xlabel={\bfseries\footnotesize\emph{Time (ms)}}
            ,ylabel={\bfseries\footnotesize\emph{Temperature (Deg C)}}
            ,grid=both
            ,every major grid/.style={gray, opacity=0.5}
            ]
\nextgroupplot%
\addplot[cyan]{rand};\label{plots:ltone}

\nextgroupplot%
\addplot[cyan]{rand};\label{plots:lttwo}

\addplot [smooth,red,thick]{rand};

\end{groupplot}

\node[text width=.5\linewidth,align=center,anchor=south] at (my plots c1r1.north) {\caption[]{Lock Time Response\label{subplot:ltone}}};
\node[text width=.5\linewidth,align=center,anchor=south] at (my plots c1r2.north) {\caption[]{Zoomed-In Lock Time Response\label{subplot:lttwo}}};

\end{tikzpicture}
\caption[]{This is a test Plot.}
\end{figure}
\end{document}