I have tried many combinations for centering following table, but none seems to work... Help much appreciated..
\RequirePackage[l2tabu, orthodox]{nag}
\documentclass[12pt]{article}
\usepackage{amsmath, amsthm, amsfonts, amssymb, mathtools}
\usepackage[algoruled,lined,linesnumbered,titlenotnumbered]{algorithm2e}
\usepackage{array}
\usepackage{booktabs}
\usepackage{color}
\usepackage{comment}
\usepackage{fixltx2e}
\usepackage[T1]{fontenc}
\usepackage[a4paper]{geometry}
\usepackage[pdftex]{graphicx}
\usepackage{lmodern}
\usepackage{listings}
\usepackage{microtype}
\usepackage{multirow}
\usepackage{pdfpages}
\usepackage{pgfplots}
\usepackage{tikz}
\usepackage{siunitx}
\usepackage{float}
\usepackage{subfigure}
\begin{document}
\begin{center}
\begin{table}[!ht]
\begin{tabular}{|c|c|p{8.5cm}|}
\hline
\begin{tikzpicture}
\begin{axis}[
legend style={fill=none},
xlabel={Time},
ylabel={Values}
]
\pgfkeys{
/pgf/number format/precision=5,
}
\pgfplotstableread
{
thread speedup
24 321.179125106
48 324.194761473
309 341.576483167
}\datafile
\addplot table[x=thread,y=speedup,color=red,mark=x] {\datafile};
\addlegendentry{a=10, b=10}
\pgfplotstableread
{
thread speedup
24 321.166812198
48 324.1978397
309 341.561092032
}\datafile
\addplot table[x=thread,y=speedup,color=red,mark=x] {\datafile};
\addlegendentry{a=10, b=5}
\pgfplotstableread
{
thread speedup
24 324.968422522
48 327.766530848
309 343.636843094
}\datafile
\addplot table[x=thread,y=speedup,color=red,mark=x] {\datafile};
\addlegendentry{a=10, b=1}
\end{axis}
\end{tikzpicture} &
\begin{tikzpicture}
\begin{axis}[
legend style={fill=none},
xlabel={Time},
ylabel={Values }
]
\pgfkeys{
/pgf/number format/precision=5,
}
\pgfplotstableread
{
thread speedup
24 321.179125106
48 324.194761473
309 341.576483167
}\datafile
\addplot table[x=thread,y=speedup,color=red,mark=x] {\datafile};
\addlegendentry{a=10, b=10}
\pgfplotstableread
{
thread speedup
24 321.163733971
48 324.186552868
309 341.564170259
}\datafile
\addplot table[x=thread,y=speedup,color=red,mark=x] {\datafile};
\addlegendentry{a=10, b=5}
\pgfplotstableread
{
thread speedup
24 322.213409372
48 325.215706756
309 342.349118139
}\datafile
\addplot table[x=thread,y=speedup,color=red,mark=x] {\datafile};
\addlegendentry{a=10, b=1}
\end{axis}
\end{tikzpicture} \\
Parameter1 & Parameter2\\
\hline
\end{tabular}
\caption{Caption}
\label{table:time}
\end{table}
\end{center}
\end{document}

centeraround the outside oftable(it just adds spurious space without centering anything) use\centeringinside\begin{table}– David Carlisle Dec 11 '14 at 16:38subfigurepackage is deprecated; use eithersubfigorsubcaption. – Mico Dec 11 '14 at 17:03mathtoolskind of loadsamsmathso just load the prior. Loadxcolorinstead ofcolor, usesubcaptioninstead ofsubfigure,pgfplotsloadstikzso that's redundant. Do not use\begin{center}unless you know, what you are doing. If you loadbooktabsyou should also use that nice package... (maybe not here). Last thing: You might want to use the legend stylelegend style={at={(0.02,0.98)},anchor=north west},legend cell align=left,– LaRiFaRi Dec 12 '14 at 06:51