我对情节感到困惑，无法解释，更不用说解决了。
一个简单的例子是这样的（还有一条从数据绘制的曲线，但我跳过了它，因为它不会影响结果）：
\begin{tikzpicture}[scale=1.7]
\begin{axis}[
xmin=0,
xmax=0.08,
ymin=0.04,
ymax=0.12,
xtick=\empty,
ytick=\empty,
xlabel={$\scriptscriptstyle \sigma$},
ylabel={$\scriptscriptstyle \mu$},
ytick={0.05,0.088,0.0947,0.1},
yticklabels={\color{teal} $\scriptscriptstyle 5\%$,\color{blue} $\scriptscriptstyle 8.8\%$,\color{blue} $\scriptscriptstyle 9.47\%$,\color{blue} $\scriptscriptstyle 10\%$},
xtick={0.0447,0.0485,0.0542},
xticklabels={\color{blue} $\scriptscriptstyle 4.47\%$,\color{blue} $\scriptscriptstyle 4.85\%$,\color{blue} $\scriptscriptstyle 5.42\%$},
every axis y label/.style={
at={(ticklabel* cs:1.01)},
anchor=south,
},
x label style={at={(axis description cs:1,-0.01)},anchor=south},
x tick label style={yshift={-mod(\ticknum,2)*1em}},
title={\footnotesize Efficient frontier}
]
\draw [dashed, color=blue] (axis cs:0,0.088) -- (axis cs:0.0447,0.088);
\draw [dashed, color=blue] (axis cs:0,0.0947) -- (axis cs:0.0485,0.0947);
\draw [dashed, color=blue] (axis cs:0,0.1) -- (axis cs:0.0542,0.1);
\draw [dashed, color=blue] (axis cs:0.0447,0) -- (axis cs:0.0447,0.088);
\draw [dashed, color=blue] (axis cs:0.0485,0) -- (axis cs:0.0485,0.0947);
\draw [dashed, color=blue] (axis cs:0.0542,0) -- (axis cs:0.0542,0.1);
\draw[blue,fill=blue] (axis cs:0.0447,0.088) circle[radius=1] node[below left,blue]{$\scriptscriptstyle mvp$};
\draw[blue,fill=blue] (axis cs:0.0485,0.0947) circle[radius=1] node[right,blue]{$\scriptscriptstyle T$};
\draw[blue,fill=blue] (axis cs:0.0542,0.1) circle[radius=1] node[above,blue]{$\scriptscriptstyle p$};
\end{axis}
\end{tikzpicture}
此代码提供以下图表：
此外，我还收到以下错误消息：
! Undefined control sequence.
<argument> -mod(\ticknum
,2)*1em
l.50 \end{axis}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! Missing number, treated as zero.
<to be read again>
p
l.50 \end{axis}
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)
但是，如果我运行以下代码（再次没有数据），一切都很好，并且没有错误消息：
\begin{tikzpicture}[scale=1.7]
\begin{axis}[
xmin=0.04,
xmax=0.22,
ymin=0.05,
ymax=0.22,
xtick=\empty,
ytick=\empty,
xlabel={$\scriptscriptstyle \sigma$},
ylabel={$\scriptscriptstyle \mu$},
ytick={0.1,0.1082,0.1401,0.2},
yticklabels={\color{red} $\scriptscriptstyle 10\%$,\color{blue} $\scriptscriptstyle 10.82\%$,\color{blue} $\scriptscriptstyle 14.01\%$,\color{red} $\scriptscriptstyle 20\%$},
xtick={0.09,0.1,0.2},
xticklabels={\color{blue} $\scriptscriptstyle 9\%$,\color{red} $\scriptscriptstyle 10\%$,\color{red} $\scriptscriptstyle 20\%$},
every axis y label/.style={
at={(ticklabel* cs:1.01)},
anchor=south,
},
x label style={at={(axis description cs:1,-0.01)},anchor=south},
x tick label style={yshift={-mod(\ticknum,2)*1em}},
title={\footnotesize Minimum-variance frontier}
]
\draw [dashed, color=red] (axis cs:0,0.1) -- (axis cs:0.1,0.1);
\draw [dashed, color=blue] (axis cs:0,0.1082) -- (axis cs:0.09,0.1082);
\draw [dashed, color=blue] (axis cs:0,0.1401) -- (axis cs:0.09,0.1401);
\draw [dashed, color=red] (axis cs:0,0.2) -- (axis cs:0.2,0.2);
\draw [dashed, color=blue] (axis cs:0.09,0) -- (axis cs:0.09,0.1401);
\draw [dashed, color=red] (axis cs:0.1,0) -- (axis cs:0.1,0.1);
\draw [dashed, color=red] (axis cs:0.2,0) -- (axis cs:0.2,0.2);
\draw[blue,fill=blue] (axis cs:0.09,0.1082) circle[radius=1] node[right,blue]{$\scriptscriptstyle I$};
\draw[blue,fill=blue] (axis cs:0.09,0.1401) circle[radius=1] node[right,blue]{$\scriptscriptstyle E$};
\draw[red,fill=red] (axis cs:0.1,0.1) circle[radius=1] node[below right,red]{{\tiny Stock 1}};
\draw[red,fill=red] (axis cs:0.2,0.2) circle[radius=1] node[above,red]{{\tiny Stock 2}};
\end{axis}
\end{tikzpicture}
我知道错误出在这部分：
x label style={at={(axis description cs:1,-0.01)},anchor=south},
x tick label style={yshift={-mod(\ticknum,2)*1em}},
但我就是搞不清楚到底是怎么回事。两个代码几乎完全相同，本质上只是数字不同，但不知何故第一个代码不起作用，$\dot 10^{-2}$右下角出现了奇怪的内容。
我很感激任何帮助。