在 pgfplots 图例设置中使用数学模式（希腊符号 \pi）

Question

由于我是 Latex 的初学者，我无法在 pgfplots 的数学模式中使用希腊符号 \pi。我总是收到错误。

\documentclass[12pt,english]{article}
\usepackage{pgfplots}
\usepackage{filecontents}
\begin{filecontents*}{example.csv}
t,$\pi$,$y$
1972,1.65,1.28
1973,3.14,1.56
1974,3.28,2.24
1975,3.04,2.50
\end{filecontents*}
\clearpage
\begin{document}
\begin{figure}[H]
\centering
\resizebox{0.75\textwidth}{.45\textwidth}{%
\begin{tikzpicture}
\begin{axis}[legend entries={$\pi$, $y$},
legend pos=north west,xlabel=Years,
    ylabel=Log]
\addplot table [x=t, y=$\pi$, col sep=comma] {example.csv};
\addplot table [x=t, y=$poil_{t}^{d}$, col sep=comma] {example.csv};
\end{axis}
\end{tikzpicture}
}
\end{figure}
\end{document}

Answer 1

错误不是由 引起的\pi，legend entries而是由\piCSV 文件头中的 引起的。只需在标题行中不使用 (LaTeX) 命令即可生成一个可运行的示例。

% used PGFPlots v1.14
\documentclass[border=5pt]{standalone}
\usepackage{filecontents}
    \begin{filecontents*}{example.csv}
        % don't use any (LaTeX) commands in the header row
        % simply replacing `pi' by `\pi' here (and commenting the first
        % `\addplot' command) will raise an error
        t,pi,y
        1972,1.65,1.28
        1973,3.14,1.56
        1974,3.28,2.24
        1975,3.04,2.50
    \end{filecontents*}
\usepackage{pgfplots}
    % use this `compat' level or higher to make use of the "advanced"
    % axis label positioning
    \pgfplotsset{compat=1.3}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        % using `$\pi$' here is not a problem
        legend entries={$\pi$, $y$},
        legend pos=north west,
        xlabel=Years,
        ylabel=Log,
        % don't show half years ...
        xtick distance=1,
        xticklabel style={
            % ... and don't use a comma as thousand separator for the years
            /pgf/number format/set thousands separator={},
        },
%        % if you want to show additional ticks without labels ...
%        minor x tick num=1,
        table/col sep=comma,
    ]
        \addplot table [x=t, y=pi] {example.csv};
        \addplot table [x=t, y=y]  {example.csv};
    \end{axis}
\end{tikzpicture}
\end{document}