\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}

\begin{document}

\tikzset{
  a = b,
}

\pgfplotsset{
  a = b,
}

\begin{figure}
  \centering
  \begin{tikzpicture}[
      font = \small,
      % A comment here
      xlabel = $f(x)$,
      very thick,
    ]

    % A simple line
    \draw (0,0) -- (1,0);
  \end{tikzpicture}
  \caption{A simple tikz example.}
  \label{fig:tikz-example}
\end{figure}

\begin{figure}[tbp]
  \centering
  \begin{tikzpicture}
    \begin{groupplot}[
        height=0.35\textwidth,
        width=0.375\textwidth,
        xlabel={$V_0$ [\si{\meter\cubed}]},
        ylabel={$t$ [\si{\second}]},
        xmax=26,
        ymax=90,
        grid=both,
        grid style={line width=.2pt, draw=gray!20},
        group style={
          group size=3 by 1,
          horizontal sep=0.5cm,
          x descriptions at=edge bottom,
          y descriptions at=edge left,
        },
        table/x = V0,
        table/y = t_RPT,
        domain=0.05:25.26,
      ]

      \nextgroupplot[
        title={Composition $z_1$},
        legend columns=3,
        transpose legend,
        legend style={
          at={(0.05,1.20)},
          /tikz/column 2/.style={
            column sep=5pt,
          },
          anchor=south west,
        },
      ]
      \addplot[cl3-1] {t_rpt(x, 437.0, 0.6*0.135294, 0.891)};
      \addplot[cl3-1, sim] table {data/instspill_m0_eps0.6.csv};
      \addlegendentry{$\epsilon = 1.2$}
      \addlegendentry{Simulation}

      \nextgroupplot[
        title={Composition $z_2$},
      ]
      \addplot[cl3-1] {t_rpt(x, 437.0, 0.6*0.135294, 0.781)};
      \addplot[cl3-3, sim] table {data/instspill_m1_eps1.2.csv};

      \nextgroupplot[
        title={Composition $z_3$},
      ]
      \addplot[cl3-1] {t_rpt(x, 437.0, 0.6*0.135294, 0.672)};
      \addplot[cl3-3, sim] table {data/instspill_m2_eps1.2.csv};

      \addplot+ [id=parable,domain=-5:5,] gnuplot {4*x**2 - 5} node [pin=180:{$4x^2-5$}] {};

      \addplot+ coordinates {
          (0.0, 0.10) (0.1, 0.15) (0.2, 0.50)
          (0.3, 0.62) (0.4, 0.56) (0.5, 0.58)
          (0.6, 0.65) (0.7, 0.60) (0.8, 0.58)
          (0.9, 0.55) (1.0, 0.52)
        } |- (0,0) -- cycle;
    \end{groupplot}
  \end{tikzpicture}
  \caption{This example will not compile!}
  \label{fig:instspill-t}
\end{figure}

\begin{tikzpicture}
  % a comment with correct highlighting
  \begin{axis}
    \addplot coordinates {
        % a comment with incorrect highlighting
        (0,0)
        (1,1)
      };
  \end{axis}
\end{tikzpicture}

\end{document}
